There are many things you use all the time in an RV. Seating. Kitchen stuff (I really like my steamer it's heating supper as I type) Fridge (Keeps the leftovers till I steam them) Bed.
And since I drink park water. filter-filtered.. Well even filtered-filtered (That's filtered twice) there is sometimes enough difference that I need to use one other thing.. A lot.. The fist tank of either the spring/summer or fall/winter season.. Fact. 80 gallons of fresh lasted me from Halloween to Thanksgiving, and the next 80.. 1 week. That was 2017. Last fall I tanked up at about 2/3 a few times (weekly for a month) and thus "Adjusted" without the after effects.