Hello Brianr.
I have been thinking about how you might be able to help your wifes mobility problem. I seem to recall many years ago seeing a caravan that had been adapted so that a person codl have assisted entry and exit with what was a step sized lift just outside the door. I have tried to find such a device but I suspect it does not conform to current health and safety reuirements for such things.
However I have been thinking of possible alternatives. I have to assume that even the 6" minimum height of the equipment tables others have mentioned would be a problem , not to mention the 90Kg plus weight of the lifts them selves.
Pursuing a number of web sits I came across bath lifts, and I have found some on Ebay This would involve setting the device up outside the caravan door, but to composite movement of up and down and swinging, and the seated position with arms may provide the range of movement to safely assist your wife. See:-
http://cgi.ebay.co.uk/HEAVY-DUTY-BATH-LIFT_W0QQitemZ300346261049QQcmdZViewItemQQptZUK_Health_Beauty_Mobility_Disability_Medical_ET?hash=item45ee083e39&_trksid=p3286.c0.m14
I don't know the weight or ultimately how piratical it may be for you but this and others may provide a possible solution.
Ultimately you need a rising step, It seem the technology may exist as a variation of simple rise and fall bath chair. These are typically rated to 150Kg, they have the necessary rise and fall distance, and some look to be quite light. They would need some development to provide the stability and the handrails for passenger safety.