I try to look for a spot that is level from side to side. But that is not always possible. Even if perfectly level, I will have to block the front of the coach up 1.5 to 3" inches. I have a 2-way level that I put on the counter next to the stove in my 31FS. I have found this spot is a good place to determine how my coach is sitting.
It realy is an educated guess as to how much you need to put under which wheel to get level. My goal is to guess good enough, place the boards and leveling blocks and drive up on them once and be done. Some times I can get it, sometimes I have to back off and try again.
I have found that my sense of level is worse after a day of driving and being tired, than when I wake up. Many times the wife and I have woke up to an unlevel coach, when we thought we pretty much had it level the night before..
I use 2x10's and those yellow leveling blocks. I wish I have the weigh capacity to add a leveling system. It would be nice to have.