I second Safari-Rick's suggestion. Keep in mind the water pump may be the highest amperage draw item in your 12 volt
system. As a result, it may act up prior to other low-amperage items ( such as lights, etc. ) when the battery is getting low.
Does your rig have a battery monitor? If so, what does it show? If not, can you take a voltage reading from the battery with a multimeter? Either would give you a clearer picture of what is happening.
Another possibility is a faulty water pump pressure switch. These are integral to the water pump in modern configurations.