Boosted, I'll wager that the new 3500 is taller at the hitch than the 2500 and you are towing nose high now. I agree with others that this is a hitch adjustment issue, I had to drop my Equalizer hitch all the way to the very bottom holes when I changed to a 3500HD to keep the TT level. I have 18" wheels also, but with 80 PSI, I don't think sidewall flex is actually an issue.
I would pull the entire rig somewhere flat where you can look at it from a distance and see if it is sitting level; I'll bet it isn't. Do some measurements from trailer frame to ground and see what you have.
D_B Travelers - 30 nights in 2016... so far. 54 nights in 2015. 2014 Rockwood Windjammer 3001W Diamond Pkg, 2015 GMC 3500HD CC DRW Duramax, Equalizer 14K hitch, TST 507rv TPMS.
"Happiness? A good meal, a good cigar and a good woman - or a bad woman; it depends on how much happiness you can handle."
George Burns (1896 – 1996)