When I carried truck campers years ago most pickups on the road were std cabs. I leveled the camper floor with a 4' level. Then leveled the fridge in its cabinet. This was in the late '60s and '70s.
I used the small " long screw on type bubble on the exterior corner of the camper right behind the driver.
My dad had a cheap made Mid States TT in the late 70s. He leveled his camper from the fridge freezer. Mom complained about all the cooking grease running to one side of her pots/pans/skillets causing burnt fried foods.
One long rainy week end while camping I leveled the trailer for her then shimmed her fridge level while dad was out in the boat fishin'. WE didn't tell him how mom fixed her out of level stove top.