It is a messy job through the bottom service door. What spec letter is the generator? The older ones had threaded fittings. The newer ones have an in-line filter. If the later, you need to cut off the original factory Oetiker clamps. Onan has a kit with the filter and two clamps, part 541-1442.
The new one is a lot easier to change than the old one. Get the 541-1442 kit the first time and then all you will need in the future is the 0147-0860 Filter. After you change it hold the stop button to prime the system and you should be good to go.
For my 30T, I had to take my unit to Cummings/Onan in Springfield, MA before moving to South Carolina. They had to drop the genset from the rear to replace the fuel filter and air filter. They recommended replacing the exhaust pipe as the Spark Arrestor was frozen in place and they could not remove it without replacing the exhaust pipe.