There is probably more deflection when you are driving down the road and you hit a dip in the pavement and traveling 65 mph. And, you hit "dips" many times traveling around.
I would consider looking for a rv repair shop with a reputation for fixing weak 5th wheel trailer frames. Call around and and ask.
San Antonio is a big town and should have one.
Tell them you are an steel structural engineer and you need to see what is going on and you need to verify any suggested repairs.
You can offer to sign a liability release so you can get into the shop area. Tell them you have a safety vest and hard hat if needed.
I hope this is helpful for you to figure out the problem.
Pat