After reading your comments I took another look. My Atwood has a line coming off the pump on the inside side. It splits, with a valve that controls the hose and another that must go to supply the TT.
I closed the one to the hose and that stopped the sucking. But I also opened the "supply" valve. Turning the pump on I got a trickle on cold side.
Now that I think of it, I don't recall ever closing it. Should it be open or closed?
And to add to the confusion I saw water pouring out of the WH outside. Sure enough, no plug. I usually keep it right there but it's gone. Can i still get water pumping inside with pressure if I bypass the WH?
I'd try things before asking, but I'd rather go out with a plan.