There are two kinds of portable generator sets (gensets). The constant speed portables and the variable speed portables.
Constant speed units are usually the contractor level generators purchased at big box stores for home standby and light construction work. The engines are similar to lawn mower engines and require basically the same maintenance. Keeping the fuel clean and the air filter clean are the most critical items along with checking and maintaining the oil level. Periodic maintenance on these units consists of checking the tightness of nuts and bolts on a scheduled basis and checking the bearings and voltage control system in the generator.
The variable speed units are the inverter type generators. They are designed for relatively quiet operation and RVers don’t usually object to them running. The engines on the variable speed units require the same maintenance at the constant speed units. In addition, the mufflers on the variable speed gensets will require periodic cleaning either mechanically or chemically to allow the unit to develop maximum power and efficiency. The generator and inverter assemblies are usually sealed and only require cleaning with a cloth and a visual check for loose wires, nuts and bolts.
Periodic maintenance that is common to both types of generators is filter cleanliness, oil levels, and changing spark plugs on a regular basis. You really need to carry a spare spark plug and a replacement starting pull rope with you. It makes boondocking much more pleasant when the beer in the fridge is cold and the spouse has lights in HER camper at night. Been there, done that.