I like the camper, it's really cool looking.
As far as the pump you can buy a pump from any RV dealer or camping world.
Camping World pumpAs far as use you just wire the pump up to 12 volts with a switch. when you get to your campsite you turn on the switch. the pump will run for a minute and pressurize all the water lines and then it will automatically turn off. When someone opens the faucet in the sink the pump will sense the drop in water pressure and turn back on again. When the faucet is closed the pump will turn off again.
If you're leaving camp you should turn off the switch in case a line bursts it won't pump the contents of the tank into the trailer without you around to hear it running.
As far as the destroyed fitting it could be one of a few things. If the fitting is above the tank it could be a gravity fill port to fill the tank with a garden hose.
If the fitting looks threaded (think garden hose thread) it is likely for a city water hookup that will provide pressurized water into the trailer without running the pump.
If the fitting is down low it is likely a drain for the tank when you are done camping.