Changing the oil can be accomplished with the generator in place. Draining is fairly straightforward, but re-filling it can be a challenge. Most people use some kind of pump to get the oil into the dipstick/filler hole.
Accessibility to perform non-routine maintenance seems to be vehicle specific, with many models of RV requiring dropping the unit to do nearly everything except change the oil.
Here is the link to the manual, in case you didn't receive one with the RV itself. It doesn't address accessibility per se, but does show the removal process for typical installations.
Onan 2800 Service Manual