As I said in another thread, to me the biggest benefit to leveling jacks is not leveling, but stability. With the jacks down, the house does not move. Otherwise you are sitting on the suspension and the house moves around whenever someone gets up at night to use the bathroom or rolls over in bed.
Our rig is a 31-foot Class C.
But then, I paid $7500 for the entire RV and it came with leveling jacks. Not sure I'd pay $4000 for them but if I was spending $20,000+ on an RV it would have to have leveling jacks.
Steve