2008 Jayco Jay Feather Exp LGT 17c tank water system help

A Jayco Exp is a separate model line 19' to 29'
A Jayco LGT is a separate model line 25' to 31'
A Jayco Ex-port is a separate model line and is ONLY a 17C
A Jayco Sport is a separate model line 16' to 21'

So I take it you have a Ex-port 17C........
That rig uses a gravity port to fill fresh water tank.
So without city water connected you just turn on the on-board pump, open a faucet to bleed air & prime pump.
Leave faucet open until pump primes and water flows.

Pump is an 'on demand' pump......runs when faucet is open and then shuts off after faucet closed and line pressure is restored (standard pump starts 30# and shuts off 45#)

City water and pump discharge feed same cold water plumbing lines. Cold water then feeds water heater for hot water plumbing lines.
No valving to swap when using one or other

Water heater has a 'bypass' valve (and cold water inlet) to isolate water heater. (6 gallon water heater)
24.5 gallon fresh water tank (30.5 published includes water heater)

Here is how a BASIC GENERIC RV water system is plumbed

Look for a valve inside on the city water supply line and close it if there is one. My trailer has a valve like that and needs to be closed when using the pump.
Also make sure your fresh water holding tank is full so the pump will get an initial prime.
How did you test the pump?