Lots of good advice above on things to try.
It also just may be that Cruiser is nervous about different "grounds". So, no matter what you do, you may have to include a lot of positive reinforcement.
If he likes toys, have some special toys in the trailer and use them to encourage him up. Put a leash on him, you go inside and encourage, and keep a consistently firm grip on the leash - this can help him "decide" to climb the steps without any fear. If toys won't work, try treats.
My Moose-dog hated the outside step on the clipper. It was just not comfortable. When it got bent during the Great GPS Adventure, I started using a seperate single step stepstool. I set it at just the right spot and he can come at it at the right angle and it fits his long body.
For our dog step (5 steps) to the cabover bed, he jumps and uses them more as a bouncing point. Bat-dog has had a couple trips where she didn't want to climb those steps to go to bed. A treat on each step convinces her otherwise.