My experience is older, but I couldn't agree more regarding the welds/structure/design of frame rail extensions. I'm willing to bet that in many cases the chassis manf. had no clue (or care?) regarding the design/execution of these?
I've been telling people for years, if they want to overload the manf's specs, they really need to get their knees dirty and crawl under the back of their coach. Arguments generally end pretty quickly at about that point. Most don't even know about these band aids. They are of the belief the chassis manf. rails go all the way back to the rear bumper?
Worst offenders back in the day were Midas, and Champion and to a lesser extent, Fleetwood. I think partly due to the fact the extensions were added after the floor had been installed? The majority of the strength came from bolts coming down through the floor!
DP's generally not so bad as the C's and gas class A's.... IMHO, of course! -Al
1997 37' HR Endeavor, 275hp Cat, Freightliner
03 CR-V Blue Ox, Ready Brake