Re: gas economy
I would suspect that your gas mileage will be 12 MPG or less. Your best mileage will probably be at 55 MPH (my truck with that engine and towing a 6000 fifth wheel got 10.2 MPG at 55 and 8.2 MPG at the posted speed limit.
A 'conversion van' is a comfortable travelling vehicle. It will get better gas mileage than a RV (because it is much lighter), but will have so-so sleeping, no bathroom and no kitchen, no water tanks and no generator.
One option if you want a bare bones RV with top gas mileage, is to find the bare (cargo) van you like which gets good gas mileage, and then have Sportsmobile make it into a semi-RV to your specifications (limited by the weight of the van, lighter gets better gas mileage but can have fewer options). Or for really good gas mileage, my dad has a Toyota Sienna van which gets really good gas mileage, and he took out most of the rear seats and has an air bed and a porta potty in there.