if only I had three friends and the drive to see a full campaign through.

im imagining starting each player out as basically just a commoner in a tank, drafted into the war, then getting bombed into another dimension. use the first fight to make clear to them how much of an advantage this steel behemoth grants them, while also letting the town they defended introduce them to a person of whatever class they wish to take their first level in, plus a little info giving (if they ask) about the tech level of the world, and where they might go to find folks who can help adapt their war wagon into a more sustainable mobile fortress/boomstick

true, but then you lose a main driving force behind the story, as well as lower the stakes, because now they can just do whatever, not having to worry about who they pick a fight with because they can always and forever either out run or out gun it

having them be forced to pick their battles based on ammunition levels, or take otherwise undesirable routes to try and conserve fuel opens the way to tension, and potential for other hooks to get dropped into the story along the way, so that once they manage to establish some way to re-up, the next story about going back and settling up with those who helped or harmed the party on their way