I used to drive 18 wheeler and when the weight police weigh you with portable scales they put one in back of all 5 axles. they are 1.25 inches high and it is almost impossible to get the truck to climb up on them. In a big truck you have to slowly engage the the clutch while increasing RPM to almost stalling the engine to get it up on them, evem with 600 HP and 2050 ft. lbs. of torque.
In a motorhome with much less torque and an automatic transmission it may not go up. Try putting the boards about six inches behind the tires on one axle and 2 inches on the other. That will get it done.