I often spot motorhomes attempting to get level by using two plastic ramps which have a total height of about four inches each. Fear of overshooting means that they raise the MH by about two inches on each wheel and the benefit is marginal at best. I use just one plastic ramp but I have screwed a three inch plank to the underside. It is bevelled on the edge so that I get a smooth run up and with help from my wife I can usually get at least six inches of height gain. (Prior to editing this read "I can get a good six inches up.") I can always spot which is my lowest wheel and simply getting that one higher makes all the difference.