Are you sure you had your water hose hooked up to the right city water connection and not to some type of tank flushing connection ?
My Itasca MH has two side by side hose connections, one for city fill, the other for tank fill. The second time I used the MH I mistakenly hooked the fresh water hose to the tank fill, instead of the city fill. I went off for a while and when I got back to my site a neighbor told me water was leaking from my MH. Only took a couple of seconds to figure out what I had done wrong. Fortunately the overflow on the fresh water tank worked as it was supposed to and nothing happened to the MH, with the only issue being God only knows how much water was wasted.
I've had a number of RVs and this is the first that didn't have a gravity feed fill for the fresh water tank and a screw on connection for only the park water connection. Just a few seconds of not paying attention in my case, since I've been hooking up hoses forever, resulted in me hooking up to the wrong connection. :o