The problem isn't just limited to TT's and 5th wheels. Our DSDP had an advertised NCC of 5,500#'s so I wasn't too worried.
When the rig came in it had 663#'s of CCC and all of that was on the rear axle, the front was overloaded by 160+#'s before we ever loaded anything or got aboard. After some discussion Newmar had Spartan replace the OEM 12,000# front axle with a 14,600# one. While on the rack Spartan tech's inspected the whole rig and wondered why Newmar had derated the rear axle to 19,000#'s. They said it was a 20,000# axle and nothing Newmar did would have changed that. Now we have a bunch of CCC but I had to go to much higher rated tires and wheels in front.
2009 45' Magna 630 w/Cummins ISX 650 HP/1950 Lbs Ft
Charter Good Sam Lifetime Member, FMCA, SKP
RV'ing since 1957, NRA Benefactor Life, towing '05 Odyssey