I can tell you the Jayco factory rims on my 3 axle TH do NOT allow for the upgrade to the 120psi tires.
I wanted to make that change, and probably should have, but elected instead ($$) to keep the wheels and put on Maxis M8008 ST radials, E rated at 80psi for 3,420lbs. I too have the 6,000lb rated axles. I hoped that the extra 420lbs rated per tire over the axle would give me enough margin to at least keep the tires round and full of air.
So far (13,500 miles into a 15,000 mile trip) so good. The tires (which I check pressure on and inspect every day) look great.
HOWEVER, my next rig/upgrade will have 17.5" wheels and G114 H rated tires.
2010 Recon ZX 39C T/H
2013 F450 W/Airbags