You will undoubtedly have to put it in, take it out, put it in, etc. etc. taking measurements, marking holes to bolt it down, etc. I had a Onan in my Cyclone that was factory installed and was a very nice installation and worked great. My new unit had gen prep so I just installed a Champion. The Champion is considerably smaller than your Onan but it was still quite a job. Here is kind of a format how I installed it and what I had to do to make it all work:
1. First set in to locate exactly where I wanted it with room considerations for other items such as my freezer.
2. Location considered for intake cooling air, exhaust venting, heat considerations.
3. Marked areas for intake cooling, exhaust exiting, and bolt down holes for generator.
4. Drilled holes for intake cooling , cut 4 inch hole in floor for exhaust exiting. On mine I had to build an exhaust manifold coming out of the generator into a 90 degree elbow 2 inch exhaust pipe that goes down through the hole in the floor and is then connected to another 90 degree elbow. The exhaust was the hardest of all of the installation. The manifold had to be welded up to make the right angle down through the floor with out touching the sides of the hole. I did not want the exhaust pipe contacting the sides of the hole for vibration concerns etc.
5. Drilled holes to bolt gen to floor.
6. The choke was on a side end of the gen that made it difficult to get my hand into to operate. I purchased a choke cable kit and fabricated a couple of mounts to make it work and installed it on the face panel of the gen.
7. Set gen in, bolted it down, had the exhaust manifold preinstalled and attached the first 90 degree angle pipe with exhaust clamp.
8. Attached 2nd 90 degree exhaust pipe and ran 7 ft straight exhaust pipe out underneath trl using exhaust pipe hangers that have rubber mounts to help on vibration. Attached a chrome end to exhaust pipe to dress it up. I ran the exhaust pipe under to the door side of the trl as I did not want the exhaust to come up under my bedroom slide.
9. I wired up a 30 amp plug to plug into the gen and wired the other end into the gen prep wiring box.
10. The gen I bought is a single phase (110 volt only) that runs up to 30 amps 3500 running wattage 4000 watt startup. My needs are met with this smaller generator where I do not do a lot of dry camping. This unit still lets me run one AC, microwave, and battery charger or other combinations of power usage. Champion generators are fairly inexpensive to purchase with different combinations of features. They have a very good reputation and are being sold through many major retailers. I got mine through Cabelos on sale.
If I were to do it again I would possible to to a little larger unit with dual 110,220 circuits and remote wireless electric start. I still have to go outside to open the front bay door up for cooling so having to go up there to start isn't that big of a deal.
11. Time to complete two days, would have taken less but cussing and screw ups added time. Cost of gen $325, exhaust pipe, elbows about $125, wiring $50. misc $25. I have a wire feed welder, oxy-actylene setup and metal chop saw, grinders etc.
12. I am adding an additional fire extinguisher and carbon monoxide detector to the front bay.
Good luck