The previous owner of my class C included a carpenter's spirit level that they used and it works well. There's a nice big linoleum floor area inside the door to check it against.
I use homemade wooden ramp blocks to level, and created a little leveling ruler that's graduated by ramp height scaled by the radio of the length of the level to the wheelbase (on one side) and track (on the other) of the motorhome. By holding it up against the low end of the level and then lifting until the level is level, I can directly read off the number of ramps needed to bring things to level.