As Ed suggests, I'd check the "latching" solenoid that's often located above or near the battery compartment. To my recollection, it looks much like the small, common old-fashioned "Ford" solenoid, with 2 small connection posts on front and a couple large ones on its sides.
Simply put, with the house Main and salesman's switch on I'd expect to find 12v on at least one of the solenoid's little posts, which should cause the closure of the inner contacts allowing higher current 12v to go from one of the large posts through to the 12v lights and such inside the coach. That implies that whether the switch is off or on, there should be 12v at one of the solenoid's large posts.
As I've entreated here before, leave the "salesman's" switch in the entryway alone. It's not designed to be turned on and off a lot, is primarily a convenience for dealers, and should be left on, period. Use the House and Coach Main switches in the battery bay to store the coach... that cuts power to the salesman's switch anyway. The salesman's switch ties to the latching solenoid previously mentioned, and either the switch or the solenoid may have lost contact integrity over time or due to overuse.
But as a preliminary, I'd check the connections at the house batteries for tightness and corrosion.
Joel