I'm at 36' overall. 26' living quarters, day cab truck. Mine is single axle, 295" wheelbase. I'm not sure how much I have extending behind my back wheels but I wouldn't want any more. I haven't scraped anything yet but I'm sure I've been very close. I leave my hitch in the lowest position when I'm not pulling a trailer and I have scraped that a couple times. My rig was built by United Specialties and I'm pretty sure they knew what they were doing. Loaded up and full tanks I'm right at 12k on the front axle, it's rating. I don't think they just guessed on that one.
I think when you see a number for a wheelbase on a tandem axle that is the distance from the front axle center to the middle between the two rear axles. At least that's what a truck dealer told me is how they measure them.
'03 Freightliner FL112, 295" wheel base, with '03 United Specialties 26' living quarters, single screw, Cat C12 430 h/p 1650 torque, Eaton 10speed , 3.42 rear axle ratio