Several possibilities
Check the line from the tank to the pump to see if it's clogged by sediment (if the tank full you'll most likely end up with a lot of water in your bilge, but at least it's clean water).Check the tank vent thru-hull for a blockage. A blocked tank vent will cause a pump to pull a vacuum that prevents anything from being pulled out of it.If you've just filled the tank, there's air in the plumbing. Leave the faucets open to spit air long enough to bleed it all out. If that doesn't work, the pump diaphragm or impeller (depending on the type of pump) has prob'ly failed. You could just replace the diaphragm or impeller, but 20 years is about the lifespan of a fresh water pump...replace it. I'd go with a Shurflo...it'll last another 20 years. 2.8 gpm is the right size for a boat the size of yours...about $80 from Defender (prob'ly twice that much from WM) and an easy swap out.