If you ever think you are going to install an ONAN, the price for the gen prep is well worth it. To install DIY you will need the generator, an exhaust pipe, an exhaust pipe hanger, and a sheet metal shroud to guide the hot air from the gen to the deck of the garage. You can fabricate the shroud with some galvanized flashing and a pop rivet gun.
It is truly a plug and play system, BUT you need to examine the transfer switch before starting the unit. Since the default position of the transfer switch is the generator, there may be a lock installed in the transfer switch holding the transfer switch to shore power. If the lock is present, starting the generator will burn up the transfer switch. (ask me how I know). I have brought this subject up before and many say that they have never heard or seen this lock. I do not know if it was left there accidentally, or if the transfer switch was supposed to have it. But anyway, be sure to look in the transfer switch to determine if a yellow lock out is installed, and test the contacts for freedom to engage and disengage.