I think this is a more complicated project than you're expecting. There quite likely is some structural framework in the exterior wall where you want your hatch. At the very minimum, I would not recommend proceeding too far without having a pretty good understanding of the sidewall construction and framing.
If it's just plain wall, by some miracle, without any studs or equivalent, cutting a hole and putting in a hatch door would be the easiest way to go. Ideally you'd probably want some sort of solid structure surrounding the cutout, such as wood blocking between the wall skins. Depending on the wall construction of your RV, this may be practical to do or rather impractical. (A wood framed wall should be comparatively straightforward, a laminated wall rather less so).
Have you considered having the top hinged as well as an end access door from the inside? You could then store seldom needed items towards the back, such as spare repair parts or tire changing tools, and move the mattress aside temporarily for the rare times you needed them.