Dog won't go into RV

I have a pound puppy (lab mix) that would not get into the cab of my old truck without me picking him up and putting him on the seat. It was a standard cab and he would just sit there terrified. I got a new super crew cab and when I opened the back door, he jumped in and laid down before I could even tell him to get in. I believe that his prior family had a crew cab and would punish him for getting into the front seat. He loves going for rides in the new truck.
Maybe your rescue had a similar life experience.