Water draining from tank overflow when hooked to city water?

First question would be: Are you using a pressure regulator? Second question: are all your valves for your water system in camping mode, rather than winterize mode? Just curious as those are the only things I can think of that might cause that to happen. Anyone else? Is the water coming out of the overflow for the fresh water tank? I'm referring to the small hole at the top of the fill. 
I wasn't aware that the two were integrated in any way.

Thanks ScottG, that was my question as well.Thanks for the photo for clarification.

That overflow vent hole isn't usually connected on the gravity flows in T@Bs. The overflow for fresh water is usually a tube running underneath. Traditionally, the vent hole is exactly where you pointed it. You can confirm this is the way your camper is configured by opening the access panel in the potty cassette and (with flashlight) look to where the flex hose comes off the gravity flow. If you don't use a pressure regulator on your city water, the water usually squirts out of the safety valve for the Alde (big yellow lever on it next to the Alde under the seat).

Usually if you are on city water and water is coming out of the fresh water tank gravity flow, that means the check valve is not working or stuck that prevents back flow when hooked up to city water. I'm not sure where that would be on your 16 MaxS. It's usually near the pump, in the pump filter screen area or inside the city water connection. 

If no one else knows where the check valve is, it might be best to call Ed K. I'm not sure your dealer would know either. Each camper on a dealer's lot is different. I just know the check valve exists and sometimes gets stuck with sand or debris, especially if you don't use a seasonal water filter, but sometimes pvc shavings can get it stuck. You might be able to try and blow out the debris with compressed air (under 50 psi).

ScottG, Makes 'ya wonder if they're plugging the leak with their finger like the Little Dutch Boy.   Would be nice to get the particulars so we could help them out though. 

Mega said:

I noticed water coming out bottom of trailer when on city connection before using regulator. Once I put regulator on so pressure was lowerered no more overflow.

I experienced a similar event and the trailer valve is just doing it's job of releasing excess pressure to the system.  Once the pressure reducing valve is installed at the water source this does eliminate the issue as noted.  Am glad things worked out for you!