Fresh water tank help

[Tank178]

Hi,
We just picked our new to us, 2005 Palomino Stampede 23SLSD and are out on our maiden trip with it. We purchased the camper from my aunt and she gave us a quick run down on how to use the systems. We initially tried to use the fresh water tank, however, we were having issues with water pressure. I read the manual and it said to fill the tank, turn on the sinks and then turn on the water pump until the water started flowing steadily. I did as the manual stated, however the water would come in spurts and stop, not to mention, the water pump was running continuously. Finally I just switched it over to the city water hook up and everything is running good with decent pressure. I did notice that the freshwater filler cap had a hole in the center (presumably for the retaining strap) and no gasket. We just up-graded from a pop-up with no on-board water, so I am still learning. Would the lack of a gasket and that hole in the cap cause the system to keep sucking in air and leading to the water coming in low pressure and starting and stopping? We will be passing by Camping World and RV dealers on our way home tomorrow, and I would like to get any repair parts. Oh, I just replaced the water filter cartridge before adding any water. Thanks for the help!