Take a look at this.
Manuals & Diagrams
I suspect the tank may have a flush built into it. My Winnebago is a different model so I cannot help other than say that Winnebago has manuals and parts lists availible online and that is where the link will take you.
It sounds like you are getting excited about your new MH and are ready! I suggest that you rein in some of your excitement until you get a chance to actually use it. Take my word on it, there will be plenty of things found that you can spend your money on after you once get started!
My 2012 Journey has Samsung TV's.
You can get covers from Camping World or most likley most dealers or online. We once left our 5th wheel in Vallejo and covered it with one of the covers that worked well!
On our Journey, with 4 slides it would appear that each slide extends at a different distance. I suspect that the brochure or manual will give you that info.
As for the batteries, ours are sealed deep cycle for the house and the starting batteries are sealed but not deep cycle. THe access is easy with each type in a seperate compartment. I suspect you can answer that from the manual as well.
Enjoy the new MH once you get it!