If you are losing water then you have a leak. (I know that is blatantly obvious but I had to say it because it is one of two possibilities.) If the pump cycles and you are not losing water then the internal pump seals
need to be replaced. In that case, it is better to buy a new potable water pump (F/W pump) because it is not worth replacing the seals
. Lots of Jabsco
, Shurflo or Groco 12 volt pumps will work. You can get a cheapie for roughly around $100 bucks. The R/V places sell them for less than the marine
stores. Your F/W pump is not going to be exposed to salt water
so whats the difference?
You might also consider an accumulator tank (pressure reservoir) which will reduce the cycling after your F/W system is working properly.