Darryl, We have a similar problem as our son, aged 31, is wheelchair bound and cannot walk at all. We are keen caravanners and take him away with us.
I have made up a set of strong steps which allows us to take him into the caravan, I have fitted a Fiamma door locking rail to the side of the door - this is not for security but to act as a hand grip to ease moving him in and out. We can, with difficulty, get him into the toilet but tend to use sites which have good disabled facilities (Caravan Club). As we go away with the family mainly in summer we tend to live in the awning but I expect that the cost of modifying a caravan is quite high and space will be lost elsewhere inside the van.