Try this spot for some of your answers;
Holiday Rambler - brochures
I would say that the Scepter is just a notch below the Imperial. But not by much, when we bought our 2008 Scepter the dealer had a 2007 Imperial and as nice as the Imperial was, it was not as clean and had not been maintained as well as the Scepter.
In my opinion the biggest difference you will find is that the Imperial has the semi-Monoque chassis.
Others will give there 2 cents worth I'm sure.