Here is a link that might be useful
I'd start by listening. Do you hear noise coming from the pump when you turn it on? if not, use a 12V test lamp to replace the pump at the water pump connection to the trailer wiring. That will check the wiring, fuse, etc. If the lamp lights
, the problem is the pump. If the lamp doesn't light
, check the wiring, corrosion of the fuse holder or connections, etc.
If the pump does make noise, the wiring & fuse are probably OK. It is possible that the pump is air bound. Some users have had success tapping the pump while running. I've even read a post where someone fixed an air bound pump by blowing in the kitchen faucet, although I've never tried it! Another possibility is the pump has a check valve that is stuck closed. Again, tapping might help.