How about a counterweight type setup where the weight (Rolling resistance) of the trailer/dolley is balanced. That way you are only fighting the weight of the boat itself when rolling the boat and trailer up the ramp, which is easy enough, unless it is very steep. Without the boat, the trailer/dolley is pretty neutral (able to be rolled up and down with equal ease)?
Too me, this would be the low maintenance solution
I am imagining a small wheeled dolley with a hitch ball, with a cable to a "derick", say 1/4 times as high as the length of the ramp run plus the flat area where you hitch up. The derrick has a 4:1 travelling block ... and 4+ X weight of trailer/dolley counterweight...
Any takers...