Welcome and first things first, there are no questions laughed at here, we all started new and understand. Now to attempt to answer your question, no you do not need to empty your fresh water tank while connected to city. I only put enough water in tank to get me from one spot to another just to have enough to flush and maybe take a shower but most of the time never use it all. You should dump your black and gray tanks before leaving a campground manly to reduce weight. That is why I only carry enough water to get me to next stop to help reduce weight.
Roger & Mary Forbes
Little Lady (Dog)
2014 F350 / 2015 CC 36 CKTS
Days Camping 2014-96, 2015-130, 2016-85 and counting