I have thought about a back up, but I really don't need it.
What I do instead is just keep a few parts on hand that I know can go out at some time, and then it is just a quick fix and I am back in business without leaving the job.
Some of these things may only go out every few years, if ever, but I keep them on my truck. Fuel pump, heater transformer, ignition switch, solenoid, pressure pump kits, small hoses, belts, couplers, check valve, brass fittings, spark plugs, etc.
These things cost much less than another whole unit and take up less space. And unless you have your extra truck mount with you, you waste time going back home to get it, especially when you are a long way from home.
Loren