Nick, IMO, if you can afford to go with a new truck, do it.
I'm building a 33ft rig (overall 43') on a Freightliner Columbia. There were 14 pages of spec sheets on the cab and chassis. Many of the spec's are specific to building it into a motorhome.
Just to highlight a few: The frame (already stretched from Freightliner) is doubled ft to back; the ft axle is rated @ 16,000lbs (vs 12,000 as rated on most used tractors), the rear gear ratio is compatible to a motorhome ie, better fuel economy (vs low ratio in typical used tractor that hauls 90,000 lbs down the road); and,...it's already equipped with rear air-suspension, stretched driveline, stretched wiring, exhaust, etc.
Then, consider 500k mile & 7 year warranty.
There aren't many dealers that have the knowlege to spec out a truck for "motorhome" cab & chassis purposes. Most of the "boys" in Elkhart get theirs through Freightliner in Springfield, Illinois (John Patkunis is the sales rep in the know).
I had the shell built @ Hawk Engineering in Jackson, Mo. (
www.hawkengineering.com)
I liked his fabrication methods & ideas better than anyone else.
He built the cage from 1&1/2" square tubing on 16" centers, uses .050" alum skins, 3/4" ply for {walk-on} roof. (1.5" sq tubing allows for ample insulation space)
Also, he uses a lot of other "better than average" materials & very tight construction tolerances which also sold me.
I'm finishing the interior and wanted the shell absolutely square & plumb,...which it is!
As for fuel milage,...It's equipped with a 450 Mercedes & Eaton 10spd auto-shift,...I drove it from Jackson, Mo to OKC, Ok (aprox 600 miles) through the back roads (and bucking a 25mph head wind) average speed 70mph,...& got 10mpg. The truck only had 500 miles on it & it's tight. I would expect the mpg to improve after it's been through the breakin period.
The Mercedes is probably the most fuel efficient engine out there at this time and the auto-shift trans is smarter than the average driver, which also makes the chore of driving much more pleasent.
Again,...this is just my humble opinion.