It depends on your mods.
I can say we tow 36K through rolling hills with stock brakes and a 32ft tri-flex-axle gooseneck equipped with brakes on all 6 wheels.
Is it safe?? Im still breathing arent I. (that was sarcasm)
I would say start low and work your way up, see what your comfortable with. The trucks will tow a lot, usually the comfort level of the operator is what peaks out before the truck does.