Also what is the general difference if one buys the b+ or we are also looking at one of the conversion vans. Would love to buy a diesel but it is out of our price range. I am looking at gas milage difference between the two
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.
I have a friend with a 2005 Trail lite b+. He gets 10 to 11 mpg. My Chinook b gets 12. This comparison is when we go on trips together, same speed, same route. The differences are: b+ wind resistance over the sleeker b. He has the Triton Ford V8 and I have the Triton Ford V10.