[JRTJH]

Cracks aren't common in any location other than the pinbox and the forward bedroom slide. While there can be a crack almost anywhere, those two locations are where almost all cracks occur.

You can check for pinbox cracks by hitching the trailer to the tow vehicle, use the landing gear to lift the pinbox/hitch several inches (don't try to lift the truck off the ground, just take the weight off the hitch). Measure the pinbox/frame distance. Then raise the landing gear to put all the pinbox weight on the hitch and remeasure the pinbox/frame distance. It should not be significantly different. If it is, there's an issue with the superstructure/pinbox welds/frame.

Then, (as you've already done) check for cracks in the corners of the slides. If you don't see any, I wouldn't worry too much about the frame integrity.