I have a Coachmen Catalina Sport 22Rk with the V-10 and it gives me about 7 to 9 miles per gallon some times more if we do not pull the toad.
What you need to be careful is not to load the rear kitchen with to many heavy items and the pantry since they are behind the rear wheels, we carry some food items in the under space of the front dinette seat and some under the couch, one thing that we do is carry a full tank of water, we tend to boondock on the spur of the moment.
The overhead bins are a bit to big so I placed a shelf mid way to be able to carry more useful items, we also have light weight acrylic glasses and Correl dishes.
The more weight the more fuel it needs to move the RV, we keep our speed at 55Mph since we tow a Suzuky Gran Vitara 4X4, the faster you go the les miles per gallon.
If the engine was well maintained it has at least another 130,000 miles left in it.
navegator