So, good news to report- I had a pro from a shop that specializes in RV crash repairs come out today. He had a good poke around the area where the pin box attaches to the 5th wheeler's frame- no sign of any flexing, bad welds, etc. All as good as gold! Definitely worth the work of pulling off some siding, (and now to replace) and paying a pro for an hour or so of labor. He says it is not uncommon to see some movement between the outer siding and the steel frame- as the siding is attached to a wood frame, which is attached to the the steel frame, that is not surprising. Wood does flex! So, next week, I will do the best I can to reinstall the siding I removed, and see if I can sneak in one more trip before it gets too cold.
I hope this experience might put someone else's mind to rest about what could have been a massive repair bill.