Robert-
First off, welcome to iRV2!
Second, you should add a "signature" to your profile. It contains whatever info you want; most people put info about their coach there, so they don't have to type it into each message. Instructions for adding a signature are
here.
As "lwmcguire" mentioned, the connection at the top is likely the winterization inlet. Your coach's owner's manual should discuss this. The "vacuum device" is the pump strainer, as mentioned.
For now, go to the hardware store and buy a 1/2-inch (NPT = National Pipe Thread) plug and insert it in that connector. This assumes the connector has a washer inside it (if not, the plug will not seal correctly).
It looks as if the tank's overflow is actually on the far left corner of the top of the tank.
It probably would help if you called Thor and spoke with their tech support folks- in case something was not assembled correctly on your coach.