For a gasser, the maintenance for the Engine and Chassis should be similar to a heavy duty pickup that is worked pretty hard. The rest should be very similar to the TT that you already have.
Tires are going to hurt a little when it comes time to replace them, especially if they all get replaced at the same time. Gas mileage is actually about the same as pulling a TT. The generator will require oil and filter changes as well.
Maintenance wise, it should not be terribly more than you already put out for your truck / TT combo now.