The toilet in my 2011 keystone Cougar will not hold water for more than about 5 min. I was winterizing my camper and put antifreeze in the toilet than came back a few min later and it was gone. Is this something I should be concerned about? If you put water in it will hold water long enough to go to the bathroom. I know this is something with the gasket, is this something that can be fixed? If so how hard is it to fix myself? Thanks for help.