If it's an arthritis issue, a couple of days of prescriptions painkillers should put her right and she would then go in readily. If it's a fear response, that's something you would have to work with her on.
Repetition is key. If there's a dog that your dog likes, that can sometimes be a help. Teach dog first to follow the treat then get it, but make it very tiny steps. Keep it fun. Change up treats and use tiny bits of really good stuff... cat food, steak, chicken (nice because it has a strong smell to a dog), but keep it very small. In training SAR dogs on agility, moving a foot can be an occasion for a treat.
Buy a bottle of DAP spray (dog appeasing pheremone), which is being called adaptyl or something like that now. But if you search amazon under DAP you'll find it, and if you go to the pet store you'll find it there in the dog area.
Once you've established this little game, try it near the ramp or steps. But before you do that, hose the ramp down with the spray.
The trick when trying stuff is to really watch the dog and see how far she's ready to go, but not push to the point where she panics and balks. Once you hit that place, nothing will work. Stay in the fun zone and let it expand, rather than trying to force her out. Dogs do learn, even very fearful dogs. But they don't learn well while stressed. You'll get farther faster taking baby steps several times, than trying to force the issue all at once.
Also spray some DAP around in the entryway of the RV.
One other thing you could do would be to set it up and spend some time in it, making dinner and hanging out. Let her stay on the ground nearby, but smell your presence in the rig and any cooking smells. Keep the door open, go in and out a lot. As she gets more familiar with the smell of the rig and you being in it, you may find she's less resistant.
