I'm well aware of A Liners, I've camped beside them, chatted up the owners. The frames aren't necessarily any better or worse than other popups, it depends on the model. But rear bumpers on many popups are not always attached to the more rigid frame members (the ones the axles attach to, so many people in the popup world caution against a rear bumper mounted rack.

The OP mentioned the skylight windows, that could be an issue.

Most trailers of this type have an A frame up front where the trailer meets the hitch. Often a propane container is mounted on that A frame. The best solution might be to take the propane cylinder and put it inside the trailer or tow vehicle and mount the bikes there. The A frame is attached to the bars of the frame that the axles attached to, so they are pretty strong.