I kinda feel sorry for your complete lack of knowledge here. What you said though is virtually impossible to experience. And maybe your math or description is off. Also presuming the fuel tank is much bigger than the 15-20 gal (why you quoted $ and not gal is also strange…) your adding every 40 miles.
Until you can provide discernable numbers and an actual detailed explanation it is impossible for us to tell you whether it’s you or your camper that needs repair.
But you DIDNT get 2 mpg unless it literally has like a 1050 Dominator carb on it a fuel leak.
I think you need to do better math personally.