I have a 28 ft tt with a big slide and the 13,500 on mine is sufficient and the warmest weather i have been in was Vegas at 111. I could see it not cutting it where the humidity is high which is where most of the responses have been from. If your staying in the west i think you will be fine.
I have driven alot of Nevada and Idaho away from the interstates and the only time i had a fuel problem was going north out of Elko into Idaho and that was in a GAS powered pickup that didnt have the range to make it from Elko to Bruneau while towing. Where in Nv and southwest Idaho are you not finding dsl ?
Our Mercedes SUV dsl has 10k oil change intervals and if you cant figure out where to find oil for an oil change or someone that can do it for for you in that time frame then you really shouldnt own one. The oil is readily available and i have a small shop that works on pickups do the service on our MB. They have no issue with finding service parts or oil.
I would tap into the brake wires and go try it. I ran wires the length of my trailer instead of trying to get inside the back wall. You dont fix sway with brakes by the way, you load right and it doesnt happen. I have been towing 2 bumper pulls for years now and its a great way to take everything we need.