If you kept all your manuals, you're 1 in amillion
Most boat owners pitch 'em without ever even opening 'em...and I'm gonna use every opportunity I can to point out the folly of doing that...'cuz the person who asks the question is FAR from the only one who reads public forum discussions. All you had to say was, you do have 'em but are out of town, and will check 'em when you get home. Once I know what models your toilets are, I can tell you whether it's ok to flush 'em with the seacocks closed. But I don't want to stick my neck out till I do know. However, from your description of it, I strongly suspect that one of your toilets--the conversion--is a Jabsco 29200. If so, running it with the intake seacock closed WILL destroy the intake impeller and housing. If your dealer advised you to do it, he gave you bad advice.So, if instead of stomping off in a huff, you'll check your manuals when you get home and let me know what you have, I'll be glad to tell you whether either toilet can safely be operated with the intake seacock closed.