Sounds like you have pretty much have the whole trip planned out.
One of the above posters mentioned "Navy Showers"? That means wet down, soap up, wash off! This cuts the water useage way down....Remember your hot water only holds 6 gallons, so it goes quick if you keep the water on while showering.
Trying to judge how much your tanks will hold is a tough one to answer as everyone uses water at different speeds.
With four people try checking levels of the tanks every second day while staying in one place for a few days, if you see the levels are filling faster then check every day. It won't take very long to figure when to dump or not!
When moving from one place to another if you use full hookup camp grounds, then be sure to dump your tanks before leaving for the next one.
Relax and don't worry as you are right there to check as often as you want too untill you get it down to a science!
Good Luck and Drive Safe!