it's also possible that it has worked its way back into the sewer compartment and is there for you to find, otherwise you could also just place a tupperware type lid over the top of it with something on top of that to weigh it down
Jazz -- Here is a link to the Thor Motorcoach Customer Care site. The site is an email form to contact Thor Customer Service. Fill in all the information to include Year, Model, and especially VIN. Explain to them that the sewer compartment closure disk has become lost and is it possible for them to send you one. Offer to pay for it, but I'll bet they will send you one at no cost. They normally respond to owner's by email within a couple of days.