comment on what I have- 2006 pleasure way lexor TD on a 2005 chev 3500 chassis 6 litre 3 speed with OverDrive.
my mpg seems to be right about 16 mpg at 65 mph cruising w/ 10% ethanol gas
the power on the 6L is perfect for the 8500 pound van.
the tranny is well matched allowing engine braking ( by downshifting) to save the brakes in the mountains- 3rd will hold at about 55 mph, 2 will hold about 45 mph.
prime mover for me getting a chev was I have GM's and know how to work on them- another brand would require a learning curve ( for example "where is the solenoid?").
if you relay on dealer/ shop service then that wouldn't apply to you.
the build quality of the cabinetry, the lay out of the electrical and plumbing systems - the entire conversion- is good
the first thing to look at is space...if you are tall you won;t want a sideways sleeper model
a tall van is great for comfort if someone is moving around while you are driving and pop-top is better for mpg...maybe not as good in poor weather camping.
enjoy the hunt- I spent months looking for mine.
lots of tire kicking
mike