1) A rubber mat is not required, but people add them to prevent sliding.
2) You don't mention tires. A lot of tires are C rated which won't get you anywhere near the max payload of the axle. Timbrens can be fine.
3) Usually old campers clear old truck cabs. You can measure to see.
4) Camper weight if "close" is usually dry weight with nothing in it. The only way to know for sure is weigh it at a scale.
'10 Ford F-450, 6.4, 4.30, 4x4, 14,500 GVWR, '06 Host Rainer 950 DS, Torklift Talon tiedowns, Glow Steps, and Fastguns. Bilstein 4600s, Firestone Bags, Toyo M655 Gs, Curt front hitch, Energy Suspension bump stops.
NRA Life Member, CCA Life Member