Rather than placing the level on a 2x4, simply put the motorhome where you'd like to park it and put the level on the motorhome floor...and then do the math. There are a couple of advantages to this: first, you get the slope between the actual points where the tires are; and second, if you have any "built-in" slope to the motorhome due to the front or rear riding higher or lower, you take that into account.
(It also helps that you have your motorhome handy whenever you want to level it, while many people don't tend to carry 2x4s with them wherever they go.)
I made a simple little ruler-like measuring stick awhile ago that was calibrated in number of ramps--I used 2x10s of various lengths with beveled ends, stacked as needed--to get level, measuring up how high one end of the level needed to be raised from the motorhome floor. Once side of my ruler was calibrated for front-to-back leveling, the other for left-to-right. I don't really use it anymore because with practice I can now eyeball it based on the level's height without needing the scale, but it was quite handy at first.