You know, the 2001 Scepter owner's manual is one of the more comprehensive that I've seen as it pertains to the chassis. It gives good general information on the cooling system maintenance but I don't see any capacities listed for the cooling system itself. They seem to list capacities for some of the other fluids, however.
Go to pages 10.265 to 10.267 for the cooling system maintenance.
If you don't have it, here is the PDF:
2001 Holiday Rambler Scepter owners manual
It does give the coolant filter part number, however, on page 10.286
If you go to this thread, the OP has an Endeavor that I'm going to assume has the same engine on the same chassis as you do and he says 11 gallons.
2001 Cummins ISC 330 coolant change
(if you don't get confirmation from somebody more in the know, we may want to ask a moderator to move this thread to the Monaco Owner's Forum