NewRW Owner the dealer has a good point, if your pump works when the tank is full then it works. I'd say you have a different issue to have fixed under warranty: the fact that you can't use all of your available water supply. Probably because the pickup hose is too short. But if the dealer refuses to accept the repair then I would bring a bucket (5 or 10 gallons), have the dealer fill the fresh water tank to capacity then count the buckets of water (thus the amount of water available) and if it's less than the advertised amount, tell him the wrong size tank was installed and you want it fixed.
2018 Redwood 3901WB Autumn Fire
3 A/C, Heat Pump, Dishwasher, Washer/Dryer, Disc Brakes, Onan 5.5kw
2017 Ram Longhorn 3500 Cummings/Aisin DRW LB CC and a cute little aftermarket fuel cap