The sunrise ramp above may help. I've seen them at some RV shows so you could try one to see if it helps.
There are companies that make aluminum ramps and platforms that breakdown for easy transport. Unfortunately they are about the same cost as a lift.
There are up to 10 ft long folding ramps used for loading motorcycles or lawn mowers into pickup that can be adapted but you would need to add a hand rail in some way to provide stability.
I would get a prescription from your DR. indicating you need a lift or ramp. Then contact http://www.spinlife.com/
To see if they can help you get the cost eliminated or reduced through your health plan/ Medicare