I have the OEM ladder and it is actually pretty complex. I am sure they had factory jigs and fixtures for making the manufacturing easier but it would be a bugger for your small town fab shop. I use mine regularly to get up on the roof to access stuff for repairs (like leaks up the center) and occasionally wiping down the solar panels.
If you don't have a ladder on the rig then you probably need to figure out how to carry one around if you have any kind of storage compartments or components on the roof. I found that the curvature of the roof line on the Travco in general makes it nearly impossible to get on and off the roof any other way but using the fully installed ladder. If you just lean a regular ladder up on the side then it can mess up your paint and or fiberglass, and if you set an A frame stepladder up beside the motorhome, it's not stable enough at the top to get on and off the roof safely without being on a steeply sloped and potentially slippery part right as you get on and off the ladder. About the only thing that can work is a curved ladder that goes up the rear like the OEM one does.
If you have the time for it, the yeah, I suggest just building the custom steel ladder at the rear and having it powder coated for long life, but it would be strictly a hobby shop job and cost prohibitive at full regular professional shop rates.