If you have fibreglass composite walls it shouldn't be a big drama, a good check of the inside is important to make sure of clearance... I would prefer to be at least 50mm from bottom of floor and not close to the wall end, think I would be keeping a good 200mm from the wall end, just to be safe, the wall is the frame and main support of the whole van, you don't want one section to flex more than the rest, so fixing the hatch frame to the wall needs to well done, cut the hole in wall neatly and precisely, then measure and cut up some meranti to be used as an internal frame, length as required by opening and whatever the width between the wall panels is with at least 50mm depth to give wall support and room to screw hatch , remove just enough styro from between the wall skin and glue the meranti trim in place, this will give the wall support and allow you to fasten the hatch in place, the hatch should just slip in place, not rattle in the opening nor need to be bashed to fit, the wall and hatch will expand and contract at different rates so nee a small bit of wriggle room .........
Meranti is stronger and far better to use than pine..
Cutting the outside wall panel, score the surface then cut with a fine tooth saw though any fractures will be out of sight as the hatch frame will cover them .....