Toilet Water Valve?
If the leak is only when connected to city water it could be a fitting or clamp at the water inlet to the toilet, or it could be the water valve that turns on the water when the toilet is flushed, because both of these are under constant water pressure whenever city water is connected. Of course, they are also under pressure when using "on board" water and the pump, but when the pump is turned back off they might only leak a tiny amount until the water pressure drops. You might not notice that small leak.
So far in three Airstreams, I have had to replace the water valve on two of them. In both cases, water leaked out the shaft that is used to actuate the valve. It was no big surprise on the '66 Overlander, but it was a surprise on our '94 Excella that last time out. It's a fairly easy fix if you can find that part (at many RV dealers) and are handy with simple hand tools, but you probably would need to uninstall the toilet to get access to change the valve. The water valve kit even comes with a new seal for when you reinstall the toilet. If you pay to have it done I would imagine it would be 1 hour labor, or maybe less if they bill by the half hour.
Wally Byam Caravan Club International Historian
Vintage Airstream Club Historian