I installed 1-1/2X3" 16 ga square tubing from the front of each basement bottom rail at an angle up to the frame rails, attached just behind the point where the frt. axle is bolted to the frame rail. Then put more tubing under the fuel tank, connecting both points of connection with bottom rails. Finally, more square tubing is installed at approx. 40 degree angles to the bottom rails. So when I go over something like a RR track the frame doesn't flex hardly at all, because the frt. angled braces try to push/deflect the frame rail and can't. Also, because the two front ends of the main frame rail don't move, the frt. axle stays fixed in place and I don't need to correct the steering wheel anymore. Also, if those angled braces want to push and move the two rails under the basement, those angled braces connecting the two won't let this happen.
I'm a structural engineer, so maybe I've given you way too much info, but hope this is understandable.

Slabman....I have read your post. I have a travel Supreme....I have done some of the X-Bracing and I am going to ad the front braces you show pictured. I talked with PP and they said they do an x brace by extending the storafe rails forward from the axle rail hangers and x-brace the axle rail hangers. Did you have this done?