I'd think you'd be better off to construct a tray than to try to find one pre-built in that configuration. Heavy duty drawer slides might be a good place to start.
Before starting on that, give some very hard thought to how you will operate the generator safely and comfortably in the compartment there. How loud will it be? How much heat will it produce, and is there sufficient airflow circulation to carry it away before it builds up too much? Where will the exhaust go, and how will you keep it out of the interior of the RV? The last question is particularly worrisome to me; carbon monoxide poisoning is no trivial matter to be slighted.
Maybe you could come up with some sort of a hoist arrangement to permit the generator to be stored in that location, but then lowered and wheeled aside for use. It would still be quite loud and not exactly welcomed in most campgrounds.