It might be worthwhile to confirm the pump itself is working properly. Either remove it and test it or disconnect the input hose from the tank and connect a temporary hose that you know is good. If you are not sure how to do this, many RV supply places carry a winterization pump kit with an inline selection valve that allows you to switch from drawing from the main tank to drawing from a hose that you stick in a jug instead. Its main use is to draw specialized RV plumbing antifreeze (non-poisonous unlike vehicle antifreeze) for preparing the RV for winter. You could use it it instead to draw regular water from a jug to help confirm the pump itself is working properly.
As stated by rrupert, there may be a crack in a hose or connection between the tank and the pump that is allowing air to enter and the prime to be lost. There could also be a crack in the pump housing itself, especially if it was not properly winterized and was exposed to freezing temperatures.
I hope this helps.
- RVs-R-Fun! :)
