Need to know what pump you have, inboard / outboard and preferably the position of your pressure switch.
Without this info there are just too many possible causes.
Most probable:
Air in system
Leaking dripping tap
Bad seal on your water connection outside
Wrongly calibrated pressure switch
If it's a pressure switch system it is probably a leak in a joint or a tap has not been turned off properly. The pressure switch can need adjusting sometimes but if the system was working properly up to now I would leave that alone and check for leaks etc first. The other likely possibility is that the water heater drain tap is leaking slightly.
Similar problem to you.. my water pump is struggling to pump water through to the taps I've checked the battery display and it's showing 0% battery life I've changed it overnight from the mains hook up and still the Same.. does the whale water pump relay on the leisure battery to pump in water ? Thought the hook up did that ..I did a test last night and the pump is pumping out water via a outlet so that seems to be fine .. I just don't understand why is not pumping into the caravan .. just what I need on our Devon day on holiday