This should be an easy question for Marathon to answer.

These are not the 4 flat halogen lights installed near the front and rear axles, correct? If so, they may be the Prevost lights. If they are, and are powered by both Prevost and Marathon, it's an issue at the switch or relay, if there is one. Toggle the one that works and listen for a relay, likely in the electronics bay just outside the driver's door. I'll look at the Prevost wiring to see if there's a relay.