Toilet water usage is minimal compared to most other uses. RV toilets do not use anywhere near the water to flush as residential ones use (they only need to flush about a foot straight down, or nearly so). Showering and dishes typically use the most water, assuming one practices reasonable conservation.
One person, in my class C, can go for approximately a week before the gray water tank needs emptying and the fresh water tank filling. That's using real dishes and silverware, washed once or maybe twice a day, and taking a navy shower each day. Obviously that sort of thing varies with tank size but it might help give a rough guide for the sort of thing you could reasonably expect. (My tanks are probably about average size for a class C, maybe a little larger than average; offhand I think the fresh is 30-35 gallons and the gray and black 20-25 gallons each, but I'd have to look at the placard to be sure.)
Dump stations may be available at some gas stations that cater to RVers, at some municipal wastewater treatment facilities, and at parks and campgrounds even for non-campers (for a fee).