I see a door, a door and a blank space, is that where you want to put the access?
If so, look at it from the other side, if there is no support beam or other obstruction,, It's a simple job to saw a hole and slip an access door, which you can get at most any decent RV place including Camping World. and bolt it in with a bit of calking behidn it to seal.
One caution.. IF you plan to paint it (If needed) to match your rig have the paint guy work on it BEFORE you cut the hole.
Home is where I park it!