The problem is that Marshall and Melinda are well-known in town for being assholes and troublemakers. No one's going to want to hire Marshall for any sort of work where they have to spend hours a day stuck in a shop with Marshall. Marshall is stuck doing grunt, semi-skilled manual labor because that is the only thing that anyone will hire him to do. Even then, I think he has to commute for his jobs, the in-town employers won't touch him. I wouldn't be at all surprised if Marshall bounces around from work crew to work crew, never managing to latch on with a steady employer. To know Marshall is to dislike Marshall. You bring Marshall on to a crew when you really need a body, and he's the first one you get rid off as surplus.
Plus, they live in a rural and very poor area where most people can do basic car repairs and maintenance themselves, or have family or friends who will do it.