Henry I reckon you have missed the Lambilevels by about 20 years!
They were made by a guy in Havant Hampshire if I recall correctly.
I used them 25 plus years ago and cant understand why they are no longer made by some some company.
Indeed I have made my own version of them, using glass tube filled with neat anti freeze.
For the new comers, they were a very large spirit level that mounted on the van front just below the window but importantly visible in your driving mirror. You then on site just crept along looking for where the ground undulations gave you a level pitch; dead simple. None of this in and out of the car to check the vans floor is level through is door.