So the question gets asked from time to time:
Why does my water heater not get hot, it is lukewarm at best?
A couple of things could be causing it:
1. Are you familiar with your water heater bypass? If not, take a quick look around for where you can access the water heater INSIDE of your RV, you want to see that back side of the water heater, take a picture and post it if you can, most RVs, have 1, 2 or 3 valves that are flipped to winterize it and it could be one of them is in the incorrect position.
2. Do you have an outside shower? If so make sure the handles are turned off on them.
Let us know what you find out. I’m thinking it is #1 above that one of the handles for the bypass is in the wrong position.
Here are some various pics of backside of water heaters and valve combinations: