Yes, dump valves for gray and black tanks. My very first rv, pulled the handle, the rubber seal was so bad, it all came out..whoosh....TMI.....
Inside the toilet bowl is a black rubber seal which should help hold water in the bowl between flushes. It is a very thin seal. If no water has been in the bowl for a long time, this seal could be dried out. Might be able to bring it back with lubricant.
One rv shower I had was a 2 part as you describe. No sealant between the 2 parts. Worked OK for me.
My tires were 15 and the tire company advised me not to go with LT tires, due to size. JMHO. I just went with normal trailer tires and the one load range uograded, to D, I think.