If you were planning several cross country trips, then 8mpg needs to be better considered for the budget. If you are planning 5-8 300 miles trips, then fuel costs are not as much % of the vacation budget. Also, budget for 8mpg. You'll be happier if you get 9mpg when you budget for 8mpg than if budgeting for 10mpg. Mental happiness - set yourself up for success. And sometimes you'll get a headwind and 5mpg, sometimes tailwind at 12mpg. The bigger issue for such low mpg is fuel tank range. No one like filling up on the road towing 20'+ RV every 150 miles.
I recommend something in the 5000 pound dry range. Something like
this KZ Spree might be good.
Another option from Cruiser RV.
