The keywords you need are "RV compartment door" or "RV access door" sometimes people use the word hatch in place of door. It is unlikely you will find a door on its own. So you do need to get the exact measurement of the actual size of the opening that is cut into the
fiberglass before you begin shopping. That cut out opening is the measurement used to specify what you are trying to buy.
Ebay is of course a good place to find them for less than the retail price from the makers of compartment doors.
RV salvage yards are another good possibility and they may be willing to ship. Just email your dimensions along with photos and do use a ruler in the photos so they can verify the measurements and also see exactly how your opening is setup.
It is possible to have a company make you a brand new, custom, access door set to match your opening size if you can't find a used or discounted one that is the correct size.