When I had a PUP and then a solid trailer and now a Class A
I used the inexpensive bubble levels (Still do) and as the original poster did. Eyeball. Sometimes I can hit level by moving the RV a bit one way or the other.
THen based on the amount of "Bubble off" I lay plank 2x12" (Tire width is about 11 inches) and drive up on them, I may lay 1,2.3 more as needed making a proper pile of it.
Once level I drop jacks since they don't work well
But with the proper planking I can usually hit level.
True story. Man with a 5th wheel trying same procedure having no luck gettin wheels on to plank properly. Told him "Get it level and I'll pick it up and you can move the planks, I can only lift 12 tons" (Got that look, You know, the "Are you from outer space" Well while he was working at getting level I took my wagon (Garden wagon) over to the proper bay and loaded up an air compressor and a 12 ton air/hydraulic jack-jack.... (he then understood how I planned on doing the heavy lifting.. With just two fingers no less)