Building Driveway Ramps

We just traded our 37’ Fifth Wheel which weighed about 13,000 lbs. in on a 37’ Motor Home which weighs almost 26,000 lbs. We are keeping it in our driveway which is black top and not totally level, so we are building ramps to drive up on to have it level when next to our home. The Fifth Wheel required tow 2 x 10’s under one side and we know from experience that even pressure treated lumber will rot when placed directly on black top for long periods. The top board was generally OK while the bottom board began to rot.

So, the question we have is relatively simple. For those who have built these ramps, what material do you use between the bottom board and the driveway?

Thanks in advance for any advice you can give.

PS: Our boards are: nothing on the right front, 3.5 inches on the left front, 3 inches on the right rear, and 5.5 inches on the left rear!