I have a 2016 Dodge 2500 Laramie with 305/55/20 with the raceme tuner in it

I haul roughly 15k pounds maybe more. I run my tuner on stock VGT and 30hp. Issue I'm having is that i can't keep my speed and in 5th gear my turbo spools very easily. Rpms don't move but turbo is spiking to 35psi. I've tried locking 5th RS run 2k but can't keep my speed.