We do use our fresh water for cooking, coffee, flushing, washing, and keeping the dogs hydrated.
I top ours off to between 1/3rd to 1/2 full. Depending on where we are going and how long we will be on the road. The only real dry camping we do is in families driveways. I'll fill up the tank when we arrive as I'd rather use my own shower and toilet than those who we are staying with. We have 3 kids, so bringing our own space with us works out well and keeps us from becoming a burden on others.
2011 Sunseeker 3170DS - 30,000 miles explored