What?! :E Take an 11 year old trailer to the dealer who will charge $100/hour for the labor he pays his minimum wage "mechanics" to fix?! And wait 6 weeks for the repair. (Sorry if I'm confusing your local dealer to most of mine!)
Really. This is simple to trouble shoot and you should be able to remove the pump with handtools and inspect for cracks at the inlet or in the pump diaphragm area. I'm betting the pump housing is cracked or the inlet hose is simply loose.
Air in the water lines can only happen at or upstream of the pump. At the pump is most common. Fresh water tank fitting is the second. Broken water line is third.
-- Chuck