3 people for 3 days might be pushing the black tank unless you use campground toilets some of the time.
Remember that it takes electricity to pump water. You don't say how many amp hours your battery bank can store, but my guess is that you ought to be conserving electricity wherever you can.
You probably have plenty of propane, but you might want to consider turning off the water heater. If you won't be showering, you could heat just enough water for dishes on the stove.
If you end your trip with a bit of water, a bit of tank space, and a bit of electricity left, you will have been successful. Going over - not good!
Have a great trip!