We currently have one in our camp and the only servicing required is to blow off the top of the burners (yearly), check the inlet water filter screen (yearly) and ensure the vent does not get blocked by spiders or other insects (yearly). This is all extremely simple to do.
Unless its not functioning properly, I think you would be wasting your money to pay for a servicing...
Owners / instruction manuals for these units can be found online here...
http://www.rinnai.us/customer-care
Dan