Cannot use my portable water source after I disconnect from water source outside the unit

Good luck on finding any piping information, even from the MFR for that. Your best bet is to remove the access panels where the fresh water inlet connects to the camper and distributes throughout the unit. Typically, the connection will be near the hot water heater then once inside, there will be a "T" connection where the water will split off to the Hot Water Heater. After that, you should see the water go back towards the wall to go through a bypass valve, which is the lever used to bypass the tank fill or direct the water into the tank fill. This is where is can get difficult to find. Many 5th wheels will have the fill line go into and behind panels that are not easily accessible or they get directed under the camper where you will have to remove belly skin panels to be able to trace.

At each one of these areas where there is a connection or corners, have someone outside turn water on/off to prevent excessive water spillage and keep any water from getting inside your unit to get soaked into wood and carpeting.

This is not just for your camper, but general troubleshooting.
Drain the water from the belly pan. Hook up water supply to shore water source. With water source turned on, allow some time, 1-4 hours, and check if water has returned to belly pan. If water is present then the problem is with the fresh water system. If no water, either it is a small/slow leak or the problem is not with the fresh water system. Next test the grey water tank by pouring some red food color into the shower or sink drain. Flush it into tank with water. Again, wait for some time, maybe over night, and then check for red water in the belly pan. It may take longer for the leak to appear as the grey water tank does not have pressure. If no water, check the black water tank with green food coloring and same technique. If there is colored water, red or green, in the belly pan, then the leak is with associated with the grey or black water tanks. If no water is present after these tests, then the leak may be very small and it may take days or weeks to have water re-appear. In any event, clear water is fresh water system, red water is grey water system, and green water is black water system. With any of them, checking the tanks themselves and all pipes and connectors to/from should find the leak source. The last possibility is that water is entering the belly pan from a roof or vent leak. Check to see if water returns to belly pan after a good rain. If so, the water may be clear or slightly dirty. If it is a roof leak, I have found the best repair is to use Peel and Seal roll roofing It is light weight laminated aluminum and self sticking. Cutting and applying narrow strips 4-6" wide, will seal any caulked area. It costs $95 (3' x 33') at Lowes. A 30" motor home can be completely covered with three (3) rolls, in half a day. I have re-topped 5 motor homes (one 7 years ago) and no more leaks on any of them. The product was developed to seal the roofing joints of mobile homes. Topping should last 15+ years.

There should be a valve near the pump or if you have a travel trailer or 5th wheel follow the hose that is leaking, up and you should see a valve.

it does exactly what it is supposed to do,shut off water when it is full

Probable cause would be a leaking check valve in the pump.

On the RV the city water and discharge from the fresh water pump are tied together. When on city water the discharge of the pump will have city water pressure on it. If the internal check valve on the pump is leaking, water will flow through the pump backwards and into your fresh water tank.

There should be a fresh water fill on the outside of the coach. If not then there is a valve that would need to be turned that will allow water to be put into the fresh water tank. If that is the case then it will be a bit more difficult to put bleach into the fresh water tank but not impossible. There will be a pump that draws fresh water out of the tank. You can disconnect the input and output lines. Connect the input line which comes from the fresh water tank to the output side of the pump. Then using a flexible line with a 1/2" pipe thread fitting, connect that to the input side of the pump. Put the loose end of the flexible line into a gallon of bleach and turn the pump on. It will draw the bleach out of the container and pump it into the fresh water holding tank. Now return the plumbing of the pump back to the original configuration. Using the valve system complete filling the fresh water tank with fresh water and allow to sit for a couple of days. Then open the low point drain on the fresh water tank and completely drain. When the coach is not in use make sure the tank is completely drained. There is also a fresh water treatment that you can purchase at your local rv parts department.

Does unit use water ONLY from pump? If so, make sure pump is actually running by feeling the pump. Is it emitting a small vibration? That is normal and probably means pump motor is running normally. If pump housing is excessively hot, pump may be locked up. If all appears normal and still no water, most likely culprit is broken impeller inside pump "Wet End" housing. Try disconnecting INLET hose to pump and check for presence of water. If none, pump is not your issue, your tank is dry. Also check pump OUTLET (pump off) for water. Disconnect hose, switch pump on momentarily. If pump discharges water here, you have a crimped or stopped up line or possibly a stopped up filter, if so equipped.

If you have been in a cold climate, there may be a possibility the water pump has frozen up and cracked. Open the lower vent cover outside for the fridge (about 14"x18") with a few long horizontal air vents in the middle. Use a coin to rotate the tabs from the two upper corner and remove cover. Locate the water line coming into the pump. If you see water in the immediate area, it may be leaking but dry the area up and have someone inside help you by filing a cup with water from the dispenser on the fridge door. If it has cracked, you will see the water start running out. It will also less when the ice maker is filling the tray. These units are plastic and cannot be fixed do you need to get it replaced. Before trying to remove the pump to take it to an RV dealer, make sure you turn off the water supply to the fridge. Most units will have a valve connected by a "T" at the cold water supply going to the kitchen sink

Where does it leak from? You city water supply line?