Who repairs diesel fuel pumps at truck stops when they break down?

@expedite_it your questions in general have been answered in some of the posts above.

There are independent services that specialize in this type work.
Here where I live in northeast Louisiana, there is a privately owned business, Perry & Sons, that does service work on fuel pumps. There are many of these privately owned services around the country. Google is your friend.

Some of the retail dealers for gas pump sales, have service personnel that not only install new pumps but also do the service work on them.
Here, there is a Citgo (a brand of petroleum products) has an office/terminal/sales/service facility here that sells, installs & services Citgo stations & customers pumps.

I don't know of any personally, but a couple of posts above have provided some basic info on schools that teach this type of trade.
My understanding here locally is that while some personnel might get formal training along with a certification, some personnel are employed by a company & learn the trade from other employees, then possibly go to a school or state facility of some sort to become certified.