Just did the first one and it was almost as easy as when I used to change oil on a CJ-7.
Pulled in the driveway just enough so the generator is still over the street which provides plenty of clearance to get to the fill and drain areas. The access panel has two latches on the bottom edge that allow the panel removal when released. The drain plug (9/16") is where depicted in the owner's manual and is slightly recessed in the outer housing.
First I checked the level since I was having issues w/ the generator shutting down but the level was correct. I placed a small plastic container under the drain and removed the plug. After it was drained, I replaced the plug. I used a suction gun to put the oil in which allowed not having to drop the spare. Since the suction gun only held about a pint, it took two goes at it to put the quart in. Used Onan's 15-40W as recommended which was on Amazon for $10.97. And I thought Mobil 1 ESP was expensive.
At least it only takes a quart.
Replaced the dipstick/cap after verifying that one quart was correct.
Glass half full or half empty to an engineer is simply a glass that was over designed.
PM me for sale info on my 2011. SOLD!