Hi Cat House,
My 4000 Onan is 5 years older than yours, but I think they are the same.
The oil drain plug is underneath, just look up from below.
To remove the access panel, there are two levers that you don't easily see. Depending on your PC, you may have to feel for them. Turn them 1/4 turn to the right and upward to a horizontal position and the top of the Onan cover then pivots forward. Then lift up the cover to get it off the bottom groove and then slowly lower it down behind the PC body, down to the ground and out.
The two black cover release levers are shown here.

I don't recall the exact place to add fresh oil but it is easily identifiable once the cover is off.
Some people accidentally loose the cover while driving because they don't properly seat the bottom of it, or latch it correctly. Make sure it's on right when you are done. Not just for protection, but also for proper cooling, the cover is a critical piece because it forces incoming air into the right places for cool operation and long term reliability.