This past summer we traveled from VA to AK, 63 days, 13,300 miles, 4 adult sized humans.
Almost $6400 averaged out to $102 per day. Mostly boondocking, we stay away from restaurants & commercial campgrounds. Includes a ferry ride, US tolls and $570 in tours/entrance/bus fees.
$3200 in diesel
$1500 groceries
$500 in state/provincial/nat'l campgrounds (would have been lower but we weren't as aggressive with boondocking on first 1/4 of trip).
No vehicle maintenance, that was a whopper service when we got back home...
It can be as "cheap" or $$$$ as you feel comfortable.