Take the same number of 2x's that level the trailer and place a 2nd set on the same side under the rear wheel of the truck. You may want to drill some small holes and drive some spikes into the ground to keep the driven wheel from pushing the board(s).
You could also carry a straight 2x6x8' and place it on edge at the point that you want the rear axle of the trailer to be after you have parked. Use a level on the board to determine how many blocks will be needed to provide a level platform. Set those blocks and back the trailer in. Just before the trailer climbs the blocks, set the same number (or similar according to terrain) in back of the rear axle of the truck. Back the trailer and truck onto the blocks.
(If the road is crowned and you want to use the leveling board, simply place a block under each end of the leveling board, then figure out how far out of level the pad is and use the appropriate number to level the trailer.)