Does tire pressure drastically affect crosswind stability?

You don't mention what kind of RV you have. Is it on the F53 and if so have you done the CHF? That will help sway a lot. Secondly, have you weighed your rig and adjusted PSI properly? I wouldn't play with PSI unless you had weights to justify it. In the end, there are some things you can do but you are driving a billboard. Wind will push it. More wind you need to travel at lower speed to reduce the impact on steering.