This is a bumper-pull trailer, isn't it?
In our previous Heartland Trail Runner, which had the rear kitchen, the water pump was under the couch (under a wood panel) along the door side wall just inside the back entrance door.
On other Heartland bumper-pulls I've read here that the water pump is under the kitchen sink behind a wood panel that will be held by two screws.
If not there, check under the bathroom sink.