regarding smaller Medium trucks,
I don't know much about the U.S but here in Oz there are plenty of converted box truck motorhomes getting around now. I've now converted 4, 5 ton sized pantecs (box) trucks over the years. Down here Japanese COE models are the best option. My last one (1992 model) was a 4ltr turbo diesel 10 speed Ford/Mazda had good highway legs cruised at 110kph (70ish mph) 3000rpm all day at around 18-20 mpg.
At this medium size, box flat chassis rails are normal with box sizes ranging from 10ft up to 20ft in 2ft increments. Headroom up to 12ft (plenty of room for a single bloke) even have cabover bed space. (known as luton peaks)
mine has a 1in steel box frame above the wooden hardwood floor (tallow wood) with alloy sheet external covering.
I reckon most over your way are gasser engines though, Isuzu, Hino's come to mind.
VT903 K125 Kenworth, 6V53 LN8000 Louisville, 4 Ford traders
In order that the labour of centuries past may not be in vain during the centuries to come...... Denis Diderot 1752