I have the manuals for the ONAN 4KY series. The 4000 series mentioned above has the following minimum compartment size:
Length (parallel to the side of the coach) - 30.5"
Width (perpendicular to the side) - 20"
Height - 14.4"
So this very popular generator should mount in your compartment, with over an inch to spare at the top.
Just be sure you provide the specified air flow.
Nice thing about 4KY is the Muffler is inside its housing also. All you need clearance for under it, is the Exhaust Pipe.
If God's Your Co-Pilot Move Over, jd
2003 Jayco Escapade 31A on 2002 Ford E450 V10 4R100 218" WB