I've watched very long, and low RVs being loaded on ferries. Tides can make those docks 15 - 20 feet different - so sometimes there are very steep conditions. The crew that load the rigs watch very carefully. As soon as they're about to bottom out, they place 2X8s under the tires - raise it up a bit - and keep doing that until things level out. (Usually no more than 3 boards.)
If it's good enough for the pros, I would think it would work for you. I agree with Mountainsam. You could build something very simple. It would mean you would have to store them until you need them. But I think it would work.
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