Did you also sanitize your water heater? If you did, it's possible you did not flush your water heater completely.
I don't understand how no water in the fresh water holding tank would create foam, sucking air. Mine never has (all 3 TT's) and it's empty 99% of the time. Did you try running water from your fresh tank unhooked to city and see if you still get the foam?
I just can't imagine your system sucking air anywhere. With pressure in the water lines, water will spew out, not allowing air to suck in. Same is true for your fresh water tank. Even if the check valve on the on-board water pump was broken, it would not suck air in, it would spew water out... back into the tank. Air and foam has to be coming from somewhere inside the system. I'd flush the water heater.