A few years ago, when I was considering a heavy hauler instead of a truck conversion, I spoke to a gentleman who specialized in singling out Semis for towing big 5th wheels. When I told him that I wanted the truck to do double duty as a RV hauler and a boat hauler, he said it was not the solution. His reasoning was just as you stated, not enough weight on the rear axle for traction, especially on slick boat ramps.
2006 Renegade/Freightliner Coronado 43', Detroit, twin screw, 12spd
2001 Ford Excursion PSD
2017 Ford F450