Was this question ever answered? I have the same question. I have a 2018 Northtrail 22RBK and want to install a ladder on the rear left side. If there Is no wood backing I would at least like to know the stud layout.
If the RV didn't come with a ladder, odds are high there is no sufficient studs/material to mount one safely. I have a ladder on my Bighorn, but I use this ladder all the time. I can use it anywhere I need...not just to access the roof. Great for clearing off slide-outs or cleaning/washing the front cap after long trip and lots of bug guts...LOL
I'll second Oregon_Camper's comments. Roof mounted ladders flex quite a bit at the roof mount. If there's no back plate, and there probably is not, mounting a ladder will just tear up your roof. Get a telescoping ladder that's gets you to the roof safely and is rated for your weight.