430 watt hours per mile at 50mph (80kph).
Aaron's 1995 Land Rover Discovery
At faster speed I would guess you might use 500 to 600 watt hours per mile.
Deep cycle RV batteries are not traction batteries, they are not designed for high discharge rates that it takes to drive a vehicle down the road and the people I've seen who use RV deep cycle batteries tend to have them last 3 years or so before their capacity has degraded to half of what they were new, golf cart batteries should be 5 years or more if you don't over discharge them or leave them sit discharged for days at a time.