Dirt in the pump
Our houseboat didn't come with a filter, and eventually some sand got into the pump. Shurflo pumps are diaphragm pumps and can pump empty without damage. Ours got sand in the diaphragm, and I had to take it apart to clean it. Not a difficult job. If you have a filter, it could just be clogged. Another possibility, is sand in the screen at the faucet. Most pumps are 45 to 50 psi, and our house pressure is 70, so it could force water through at the higher pressure. Just unscrew the end on the faucet to check. I've been amazed at how much sand is in our water system at home.
2009 Roo 21ss + 2007 Superduty 6.0