What we did for awhile was similar to AP by using a level, but our level was a 4' that I placed between the front landing legs up against the bottom metal of front compartment. Bit of a pain checking and rechecking, but it got the job done for side to side.
Then placed level up against underside of left frame rail to get front to back. Never once did I go inside of camper to check levelness. No issuses at all when all done.
At home, repeated the same process when parking besides the garage. After I installed the big bubble level on 5er hitch, that had to be adjusted so it read level. Then added a small stick on/screw in level on left side by switch for front to back. Both levels were installed at the house as I knew 5er was already level.
We carry a 2x 10 x 6' and 2 x 10 x 5' for main leveling and then several 1x6x 12"for the fine tuning if you will for the side to side. Can get this done in short order once you get used to the readings on the big bubble level.
Front to back will take some getting used to depending on ground level itself and if you extend the front legs within themselves or not. This step screws up the Mrs all the time. But we normally just place a 2x4 or 2x6 (6"-8" long) under each legs to spread the weight out. Plus some 1/2" and 1/4" x 4"-6" to eliminate any gap between leg and the said 2x4/6.
Sounds like a lot of steps, but once you get use to it, can get the job done in no time flat.
Wife has said the next 5er we get will have the auto-level up and power awning.