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.