There are civvy portable steps available, for helping warehouse workers to get up to shelves. Some of them don't weigh much, and may work for your app. They are light enough for 1 person to put in the back of the truck. You will have to lash them to the side of the truck (for loading), just for the safety and confidence of your passenger.
If the mobility is too compromised, a crane or lift may be in order.
I know that when we do parades, we quite often have to back the truck up to a loading dock, so that our older Veterans can easliy get into the back of the truck. It isn't the same as riding in the companion seat, but they still ride.
If you can give us a better idea of the requirements, and the usual abilities (as they always change daily) , we might be better able to advise you. If you wish, feel free to PM me.
No matter what, please keep us updated, on your solution. More and more people, who would like to come with us, will need more assistance. All ideas are needed.
Thanks for bringing up this problem. I am looking forward to the proposed solutions. They may not work in every case, but every one is different. If just one can help 1 person, well.. BONUS!!!!