You can call Winnebago Towables Customer Care at (574) 825-8052 and ask them for this kind of thing. But I’ll hazard a guess that it’s not possible. RVs are not built like houses with regular framing at set intervals and built in backing for attaching things. In general framing only exists where it is needed for the installation of some OEM component. And since there is no factory installed ladder there is no backing anywhere for you to install one.
My suggestion is a foldable or collapsible ladder in the back of your tow vehicle.
Diagram shows wood backer boards on the righthand side (3) spaced backers that i assume can ce used to mount a ladder. Customer services responded in 2 days on my request for a rear wall schematic