I've used the stick-on bubble levels that I got at a camping store for something like $6 a pair. One is installed on the front of the camper where I can see it out the rear window as I back in. The other is stuck on the roof edge near the front so I can quickly check it as I crank the jack. If you have a smartphone, there are apps that show the angle of tilt of whatever the phone is on. They're remarkably accurate. I'd use that method if I didn't already have the stick-ons.
It's never too late to have a happy childhood!
Lee, WU0V, and Courtenay, N0ZDT
2011 Rockwood A128
2000 Silverado 1500 pickup
60W solar system
2000W inverter generator