One option: Would be to purchase 2 or 3 gallons of RV antifreeze and bring them to your inspection. Your total investment would be under $10.00. Most RV's have a winterizing port where the gallon container can be used to feed the on board water pump during the normal winterization process. All that the dealer would be doing is adding RV antifreeze to a system that is already has it in it. By pressurizing the plumbing system with additional antifreeze you would be able to demonstrate the integrity of the sink, shower, and vanity faucets (and drains) and the toilet. The hot water tank is not winterized with antifreeze (it is just drained) and as such the tank itself would remain the only component on the pressure side of the plumbing system that could not be tested using this process. While a problem with the integrity of the hot water tank is not out of the real of possibility, it's an unlikely to be a concern. Both the grey and black water holding tanks would have some of the RV antifreeze inside them during the normal winterization process, and if not in tact would show antifreeze leakage. That leaves the on board fresh water holding tank as a plumbing component that could not be integrity tested. Like the hot water tank, the fresh water holding tank is simply drained for winterization.
2006 Winnebago Journey 36GSE; 2014 CRV; Blue Ox; SMI AF-1