Welcome to iRV2.
My 7.5 the air filter is accessed at the front / bottom ( opposite end from the fuel/electrical hook ups). If you don't have a manual for your Onan, there are some in the " Files " drop box under "electrical".
The " Files " drop box is in the dark blue bar , across the page under the Google search , and your welcome message.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.