Checking black tank capacity

Isn't the black water tank only 6 gallons total? That would mean after your 3 days 2 nights you had only used 1 gallon of its capacity. Thinking about 2 people, 75% of the time, maybe that's reasonable. My only extended trip so far was 4 days, 3 nights, and although it was a full hook up site, I did not leave the dump valve open. My wife used the on board facilities exclusively and I used them partially. To better rinse out the holding tank we added about a gallon of fresh water before dumping and we were nowhere near filled. We also showered each morning and used the sink as needed we did not fill the gray water holding tank either (I watched for any water pooling in the shower area to indicate it was full). I think the real limitation for us if boondocking will be electrical power (if we cannot use the a/c that is a deal breaker for me) or the fresh water 11 gallon capacity. So far we have only camped at full hook-up sites.