On my Lazy Daze, the ones without the generator option had a compartment as you describe. Mine, with the generator, is open at the bottom and the generator frame is bolted up to the van frame and camper floor. like it's hanging down. Theie is no compartment. The cover is a meshed screen instead of a solid door to allow airflow.
I have seen campers where more batteries and extra solar panels were installed with a large inverter instead of a generator. Since I don't run the generator just for A/C, all my needs would be met with a 6-battery, 450W solar and 2000 watt inverter set up.
I bet it would cost less than installing a new generator.