@Tunnel Me Daddy

So a few hours after my previous commentary i got this mod running smoothly on an existing singleplayer save, in case anyone wants to know what i did i'll let my steps below (If OP feels that this is unnecesary just delete this comment) :
1- Create a new sandbox world with this mod enabled
2- Spawn the vehicles (You can use a menu for this) and make sure you enter to every single one of them for the interiors to generate
3- Go to the folder on User/Zomboid/Saves/Sandbox/NewSave and copy the map_ver.bin from there
4- Next go to the save you want to enable this mod on and paste the map_ver.bin from the previous step
5- It should now work, you can check by spawning an RV and entering.