I have a 2004 New Vision Sportster that I've owned for 2 years. A crack has formed on the outside wall from the bedroom slide. The crack opens up when all the weight is on the 5th wheel and relaxes back to near normal when the weight is on the landing gear. I've removed all the tin work from underneath the over hang as well as the siding in front of the generator and LP compartments. The 5th wheel pin moves 2" before the weigh is removed from the landing gear. I thought that I would find a cracked weld, twisted tubing, or metal tearing. But I can't see where the flexing is taking place, much less what needs to be reinforced. According to KZ the frame was made by Leland, who is now owned by Lippert. Neither was any help. Anyone else heard of anything like this or know what to reinforce?
Google "fifth wheel frame cracks" and you will find plenty of faults. It would be best to discontinue use and find a qualified repair facility that can open up the affected area and reinforce. If you want to do it yourself, there are plenty of photos online that might help you.
Unless you are an experienced welder / fabricator this is one of those repairs best left to the pros, as you have found finding the problem area can be tricky & making a proper repair can be just as difficult.
2018 Durango 318RLT '20 Ford Lariat F350 4X4 Former RVIA Master Tech, ASE Master HD Truck Tech.