[RollaMo]

Most "owners manuals" are really generic, and talk about options that may not be on your specific trailer model.

My guess, is since you say you have a gravity port to fill the fresh water tank, you may not have any valve that allows the fresh water tank to be filled that way.

Also, you will have a fresh water tank drain valve on the bottom of the tank to allow you to drain.
Look for that under your trailer very close to that gravity fill port you mention.

Of course pumping like you are doing works also, and will help flush nice clean sanitized water through your faucets.
Not sure if it is hard on the pump having it run continuously that long.