Beginner’s Guide to DEF for RVs
Video showing how to use Diesel Exhaust Fluid and why it is important.

By: Peter Holcombe

If you are a new diesel RV owner, one term you may not have been familiar with before reading your manual is Diesel Exhaust Fluid (DEF). Simply put, DEF is an additive that goes into diesel vehicles to help reduce air pollution.

In the video below, Winnebago Revel owner and long-time RVer, Peter Holcombe shares everything you need to know about DEF, including:

  • Why is DEF important for your diesel RV?
  • Where do you find DEF?
  • What happens if you don’t refill the DEF in your RV?
  • Where is the DEF tank located and how do you fill it up?


Comments on this post are moderated, so they will not appear instantly. All relevant questions and helpful notes are welcome! If you have a service inquiry or question related to your RV, please reach out to the customer care team directly using the phone numbers or contact form on this page .

User commented on November 20, 2022 1:35 PM
When filling from a dispenser at a service station how do you know when full so that you don’t over fill and spill? Does the dispenser nozzle have an auto cut off like the fuel dispensers?
User commented on November 20, 2022 4:19 PM
Yes, they do. Mostly. See my paper towel trick in my comments.
User commented on November 22, 2022 7:44 PM
Hi, that is a great question. The bulk DEF nozzle does have an auto cutoff like the diesel nozzle. It makes it really easy to fill.
User commented on November 20, 2022 4:17 PM
The average Sprinter DEF tank capacity is 4.7 US gallons or 18.0 litres. Before filling your DEF tank, fold a paper towel in 4, poke a hole through the middle of the folded towel, and press it down around the threaded DEF fill pipe to catch any overflow or splashing. Lastly, if you use the boxes or other containers, verify that the seal is original from the factory, and hasn't been tampered with in any way before putting it in your DEF tank. DEF resembles water, and some people have filled their DEF tank with water, after someone else bought the DEF, used it up, refilled the container with water, and returned it to point of sale, for a refund. Seriously, be very careful when using the off the shelf stuff. The truck stops stuff should always be pure DEF, and because so many diesel trucks use it, it's more likely to be fresh. DEF can age out, although it would have to be exposed to heat and light for at least 2 years or more to do so. MK.
User commented on November 20, 2022 4:17 PM
DEF at the pump is much cheaper than a bottle or a brick.
User commented on November 20, 2022 9:37 PM
Is it possible to post a transcript of the video? Thanks!
User commented on November 22, 2022 12:34 PM
Thank you for your comment! I'm sorry, I don't have a transcript on hand, but if you click "CC" at the bottom of the video box above it will turn on subtitles/closed captions. (Note: there are a couple of transcription errors when he says DEF, but otherwise it should be accurate). Hope this helps! -GoLife Editor
User commented on November 21, 2022 7:34 AM
Very informative video. Nothing mentioned about using only fresh DEF. Some DEF containers have dates of manufacture and others don't. I've read that to only use DEF that is not over a year old. This is a very good video and the speaker is precise and clear to understand. Thank you.
User commented on November 21, 2022 2:43 PM
DEF is NOT as easy to deal with as Peter shows so easily in his video and there are some other things I've been told AND learned from experience. 1) Don't store DEF for a long time. It isn't like a quart of motor oil that you can store for five years. Use it up in a much shorter period of time. It has a shelf life and is not intended to be stored. 2) The fill hole on my 2019 View appears to be the same size as on the Rebel in the video. The hole has proved to be too small and the location is further under the hood than in the Rebel. Two different times I've tried to fill up at the pump, and the gas station nozzle did not fit into the View's DEF opening. Another time the pump nozzle barely fit inside and it spit and sputtered DEF the entire time I tried to fill it - what a mess! 3) I've yet to fill DEF fluid from the 2-1/2 gallon box and NOT made a mess. Be prepared to spill it on your vehicle and potentially your clothes as well. 4) Get a funnel that stays put! Peter was having to hold his funnel in with one hand while filling with the one gallon bottle. Also pour slowly - it will fill up the funnel and overflow, or fill too fast into your DEF reservoir and go all over the place. 5) One gallon containers, while a lot more money, have proven to be far easier to fill with and overfill/spills are minimized. 6) Wipe off excess overage from around the DEF fill location in your vehicle. Use a junk rag or paper towels and deposit them in your gas station garbage. As Peter mentioned, the fluid exposed to air crystalizes and leaves a mess around the rubber catch area below the DEF fill tube. 7) Did I mention funnel? Be sure you have one and keep it clean and ready for your next filling. Good video Peter! Should help Winnebago users better understand.
User commented on November 22, 2022 2:33 PM
Sounds like you've just had some bad luck with it. I've had no problems from day one, 4 years ago. It is easy to deal with if you're careful, and plan ahead. BTW, all DEF pump nozzles and fill pipes are a standard size, as are the correct DEF "at the pumps" equipment, so I'm surprised you say yours is smaller? From the Cummins Diesel website. Q. What measures have been put in place to prevent diesel from being pumped into the DEF tank? A. The standard nozzle diameter for dispensing DEF has been designed at 19mm versus the standard diesel fuel nozzle diameter which is 22mm. In addition, the tank cap for the DEF tank will be blue to further differentiation from the diesel tank. This will help people to NOT put diesel fuel in their DEF tanks, although a really determined RVer could still put DEF in the diesel tank. Better luck with it. MK
User commented on November 22, 2022 5:57 PM
Thank you for sharing your experiences and insights! I've reached out to Peter to have him hop in to address some of these questions as well. -GoLife Blog Editor
User commented on November 22, 2022 5:00 PM
Should you empty the DEF tank annually and refill with fresh DEF? Will it freeze up when parked over the winter?
User commented on November 22, 2022 8:03 PM
It’s a good idea to use fresh DEF but I don’t worry too much about it. I’ve burned two bricks in the last month. It could be a good idea not to fully fill your DEF tank at temperatures lower than 12F (DEFs freezing point). This gives a little room for expansion. I’ve had my Sprinters down in the -20s and never had a problem with starting or freezing fluids. Once the engine runs for a short time, it will be warm enough to melt the DEF tank for the next regeneration cycle. These are great questions. -Peter Holcombe
User commented on November 23, 2022 12:55 PM
Good video
User commented on April 1, 2023 4:37 PM
Why is my low def light on when we just filled it?
User commented on April 5, 2023 1:19 PM
Thank you for your comment! To troubleshoot what may be going on, please reach out to the customer care team directly at (641) 585-6939.
User commented on April 30, 2023 3:57 PM
Does the DEF "system" on a coach have a DEF Filter? If so, where is it and how often should it be replaced?
User commented on October 23, 2023 11:11 PM
I’m not that much of a internet reader to be honest but your blogs really nice, keep it up! I'll go ahead and bookmark your website to come back down the road. Many thanks