Looks like an option would be some SumoSprings (about $170) which are a closed cell polyurethane system that doesn't require maintenance or air. I could git a pair and install one (maybe chopping it down) on the trailer wheel suspension and use the other one as the hitch dampener.
SumoSprings Custom Helper Springs Kit - Rear Axle SuperSprings Vehicle Suspension SSR-202
![](http://www.etrailer.com/Merchant2/graphics/00000001/pics/S/S/SSR-202_1000.jpg)