You should have Trace/Xantrex SW4024-MC2.2 in your Marathon. You could use the Magnum MSH-4024RE which also has two ac inputs for shore and gen like the ones you have now. You will also need the ME-ARC remote for each one, and the AGS-N network autostart module. They made the system modular instead of everything being in the inverter itself like the Trace/Xantrex unit.
However, before you go running to change that inverter, go check the white button breaker on the left side of the inverter and make sure it is not tripped. That cyclical on and off is usually indicative of that breaker tripping, or even being defective. If that is the case, the breaker can be replaced without having to change the whole inverter.
Another option is that I believe our parts department has 2 rebuilt units in stock that were repaired and checked out at Southeast Power Systems of Orlando, our closest Xantrex repair facility while the parts/boards were still available. You can call Dan Hartley if you want to go that route at 727-420-6805.
Let me know if I can answer anything else for you, and what you find.
Ben Cummings
U.S.A. Luxury Coaches, LLC
4501-B Ulmerton Rd
Clearwater, FL 33762
USALuxurycoach.com
813-830-8619 (cell)