There are a couple way you can add water to your on board fresh water when you do not have a gravity fill port, but only the port for a city water hook-up, (and this is assuming you have the outside blue coiled shower-sprayer hose with an outside faucet).
All you need to do is attach your winterizing hose, flip the valve to "winterizing" and start sucking the water from your water source, be a bucket, or a jug, or a bladder in the bed of your truck.
The blue coiled hose now attaches to the faucet as always, and the other end attaches to the city fill. Flip your valve to city fill, and turn on your water pump.
Water will draw from your water source through the winterizing port. It will circulate to the outside shower (coiled blue hose), and then flow into the fresh water tank. Depending upon how much capacity your water source container is, will determine how many trips you'll need to make to get water to fill your fresh water tank in your camper.
The other option, is to purchase stand alone water tank, or a bladder that fits the bed of your pick-up truck and use a transfer water pump to pump the water into the fresh water tank, which would be the same as connecting to a house water hose.
I've used both methods, but only the first in experimental mode, just to be sure I could do it in a crunch. The second method, I've done, probably hundreds of times: