Onan marquis 5000 model # 5bgmfa26105f runs for a few minutes and shuts off. let it cool off and it will start again. whats the problem?

There are many possible reasons from stator output voltage to sensing, to fuel to control boards. A good place to start (as the unit does run) would be with the fuel system, if it has sat a long time it won't help. Also the fuel filter. try conencting to an external fuel tank versus the vehicles, as sometimes pick up tubes in main tanks can be an issue. Also check the correct oil level and viscosity / and oil pressure switch. Failing that I recommend a service center, as there really are so many causes, I cannot list all on here.