Paul, your question about the grey water dumping is a firm 'yes.' You cannot dump grey water as far as I know on any public lands such as national forests or RV parks, etc. Very few areas, if any, would regulate shower water on a ten acre site. You are dumping shower water or dish water on your own land. Blackwater doesn't enter into this same situation. You are building a house; surely you have a septic tank you can pump into or your public sewer line has been set up in advance? This a little article that may be of interest:
https://greywateraction.org/greywater-codes-and-policy/
And as far as how often to dump an 88 gallon black tank;
that depends. Are you going to have people in for a drink? Maybe for lunch? Are you going to require the men to go behind a bush? Who will be in the RV who realizes how little capacity you have? If it is just the two of you then maybe four days on the outside.