Check your battery cables
We were coming through St. Louis on our return from Sturgis, SD when our coach quit similarly to what you described. It took out engine, power steering, brakes, flashers and who knows what else. I muscled it to the shoulder and used the emergency brake to stop. I didn't even piss anyone off by slowly crossing 2 lanes on the freeway. It would not restart so we were stuck there.
We called the Good Sam road service and since it was a Sunday they were having trouble finding someone to rescue us. While waiting for a call back I decided to look myself. It turned out to be a broken battery cable lug on the coach battery. I clamped it on with a vice grip and we were on our way.
Mark, Bonnie & Maverick the Rat Terrier
'99 Fleetwood Pace Arrow Vision