I started with the rv carpet step covers and that did the trick for my chicken dogs who don't do stairs AT ALL. If that wouldn't have worked I would have tried blocking in the backs till they got used to the stairs.
Yummy treats of course help. Also, use the same idea that they use in agility training to build the behaviour - step by step (lure them up one step, then back down, repeat as necessary until they are all the way into the trailer.)
FWIW - Spike, who is the same size as Jimmy, feels he has to back up and take a run at it. I'm sure he could manage otherwise, but this works for him! I should probably take the time to teach him to take it slowly... next summer.