We also have a Grand Design Transcend. Not sure if it's like yours, but our water connects through the floor and into a universal docking station. There is a lever that enables you to switch from city water, where the water goes directly to the faucets, or to fill the fresh water tank. Once connected, you should be able to lock your storage compartment door. You may want to turn off water at the spigot if you will be away from your camper for any length of time.
I found the connection a bit tricky at first. I couldn't seem to secure the hose tightly enough and usually had a drip. Never a good thing to have water leaking in storage. I was also concerned with things entering the camper through the hole.
After some experimenting, I found a setup that seems to work for us. I documented it with pictures at: https://traveltrailsail.com/rv-drinking-water-safe-hose-connections/
Hope that helps.
