Full timer here with years of practice but if you start out 'thinking' of water conservation, you should be fine. Won't take you long to figure out what your needs will be.
I've never seen much difference in ride or fuel economy between a full or empty water tank. I tend to drive with it between 1/2 and 3/4 full and that gets me by as I hop from RV camp to dry camp to RV camp. I always ask about water quality when I arrive at a camp, and check for clarity and taste before using. Plus I have a filter under the sink for drinking water. Not unusual to forego connecting to a campgrounds water if it's not up to par.
I can go 10 days easily on 60 gallons. There's just me, I take a normal shower once every 3-4 days though I do sometimes need to take Navy showers for a while, can go a week with only washing dishes once by being conservative with dishware, and sometimes use paper or foam plates if I'm in a waterless area for a long time. Always use the flush toilet normally, and do not fill the bowl after use (there's always some water there after flushing).
So on average 6 gallons a day for one water conservative person.
Longest I've gone without a tank fill is 21 days (2.8 gallons per day), and I still had some water left when I filled up.
So as you can see from these posts, there's as many procedures about water conservation as there are people. Your best bet is to fill the tank, and try to be conservative about water. One big thing is that since you're camping, do you both really need a long hot shower every day?
Good luck and have fun.