I bring a 2’ construction level and put it on the a frame as close to the box itself. When that is good I open the roof and put a bullseye bubble level as far inside as I can see to verify it is good enough. I found on my camper my roof isn’t fully flat and get a different reading. My back bumper my reading is a tiny bit off from the front. Not sure what the deal is there so only rely on where my a frame is. At least with my camper it is more true to flat then anywhere else. My old camper had stick on bubbles on the side of the camper and one in the front near the top. However I could never trust them alone as every site seemed to get them confused. Where they showed to be level but when I put the bullseye level on the floor I’m anything but level. Never did figure out why. Every camper is made differently and every campsite is too. so the key is to find the location on your camper you get the most accurate reading with every trip. The important thing is to make sure the fridge is good enough so where ever on your camper matches that level you should rely on. The one time I forgot my level I used a flashlight on the floor of the camper. If it rolled I wasn’t level etc. I also used a cell phone app. Not sure I would trust either method 100% but I surprisingly didn’t feel like I was sleeping on a hill so it was good enough for me. I didn’t use my fridge then as I just didn’t trust the method enough. Let me tell you some sites are way more challenging than others and I can almost never be dead on.