Re: New member - some questions :)
The only thing I would add is be skeptical that one vehicle may get more mpg than another especially when built out as a relatively heavy Sportsmobile-type rv.
Yes, there are small differences, but in my non-engineer opinion/knowledge/experience it takes a certain amount of BTU's (or other appropriate unit of energy) to move a certain amount of MASS. You can accomplish moving that MASS with a small engine sipping 1/2 the gas per cylinder but spinning at twice as many rpms (and using a many-geared transmission to leverage that less powerful engine to move that MASS) or use a larger engine which guzzles 2x the amount of gas per cylinder (and which may not need as many gears to move the same MASS). It still comes down to the same amount of fuel because the laws of physics (and I have never taken a physics class) in my experience state that it takes a certain number of units of energy to move a given MASS regardless of the engine/transmission combination.
Recent case in point. My neighbor who owns four IDI diesel Ford pickups used for business just bought a Dodge 2500 with the 6.7L Cummins - for the reputed increase in mpg - and he is disppointed because the way he uses his vehicle he gets only a little better mpg over the 20-year old paid-off 7.3L IDI Fords but does get more power, monthly car payments, higher registration fees, higher insurance premiums, DEF, a turbocharger and other complexities and more expensive maintenance with the newer 6.7L Dodge.
And be doubly skeptical of our gov't including the published EPA fuel mileage stats per vehicle. Remember last year when Honda lost in Small Claims Court because they way over stated their mileage claims and the court would hear nothing of Honda's argument that the "EPA made us publish them." It was overturned on appeal but the principle stands, the EPA is overstating the improved mileage benefits of newer vehicles.
And oh yeah, if you like seeing the country around you where you park your van, get a window van. It comes with rear air and heat and you don't need to install windows or walls. And if you add a fixed fiberglass top, you can put all your storage up there. Anyway, that's my two cents.
2002 E350 ext.; 160K; 7.3L; 4R100 (w/4x4 deep pan & filter); 4x4 conv. w/2007 F250/F350 coil frnt axle (oppos. dual Bilstein press. shocks cured DW) diff chg from 3.55 to 3.73 (bad!); BW1356 t.c. (bad!); LT265/70R17/E Michelin LTX M/S2; Engel MT60 Combi Fridge-Freezer; 4 BP 380J pv panels; Auragen 5kw AC gen. in top alt. position; Webasto Dual-Top; Voyager top. 1995 5.8L EB Bronco, bone stock.