I worked in mgt/engineering for a company that manufactured torsion axles in Ohio, I think mostly for AirStream. Learning how the rubber is nitrogen frozen and the axle assembly process was quite interesting.
My job was moving a hub manufacturing business to the site from Tenn. I set up a lot of machinery but had to abandon some old high production equipment in favor of PC. A drunk for the GM boss made that get old after about 6 months. GM got mad when 25k ag hubs had about 25% that I could not get within a rather tight bearing concentric tolerance and had me scrap about 150 of them...75% being well within tolerance. Ticked me off and I bought a bunch at scrap price myself and quit, moving back to Indiana. WW2 vintage machines we moved that they insisted on using just wouldn't do it, chucks were worn out.
Those 450/80/20 Sand Trail on the 13.19 bc rim that sold cheap a year back fit perfectly. So I have the about 8 sets of the 4" axle, 25k hub, wheels and tires. Just need a paying project for them.
Gizmag or someplace had an ad for an $80k camper with some really cool stuff (yea right). Here is one I really like that might not be impossible to take some design from.
https://newatlas.com/adak-camping-trailer/33280/
These are very robust axles and fastening additional frame to them is really easy....JUST DON'T WELD....there is highly compressed rubber about 2' from each end. I have a couple pieces of 3x5x3/16" wall that might make the front frame but a solid piece of 3x3x1/4" for the entire length would be nice too (don't want to buy it at a cash poor time).
Thanks for the feedback Tinstar.