Great question. I'm still new to TT's, but per advice on this forum, I travel with 10-15 gallons in my fresh water tank (for 'just in case' circumstances, as we haven't done any boon-docking yet, and weight isn't an issue).
If I know I'm going to be camping within a 1-2 weeks of the last trip, I'll leave whatever fresh water is in there. If I'm going to be home longer than that, I'll drain the fresh water tank and then refill the day of the next trip.
I plan to sanitize the system every few months. I should note that we don't use this water for drinking, other than the little bit you'd get from brushing your teeth. Not that I wouldn't drink it, it's just that we've always carried bottled water during our tent camping days, and now we do the same with the TT.