I do our coach with a gallon, maybe gallon and a half. How much you use will depend on coach equipment (like ice makers and washing machines which will add to total by quite a bit), as well as hot water heater by pass kits and source selector valve for water pump. After that, your experience doing the job will be a factor. Safe bet you won't be using as much the 4th or 5th time you've done the job.
2 biggest tips - make sure the water heater is bypassed prior to pumping anti freeze, and do not put anti freeze in the fresh water tank.
1997 37' HR Endeavor, 275hp Cat, Freightliner
03 CR-V Blue Ox, Ready Brake