How fun for you!!! We have a 2015 351, and have had it about a month
For us, the slide on the TV bracket isn't a big deal, as we don't have any kids and don't watch TV much when we are moving, but it would be nice to have while you were parked!
Not sure on the inverter - ours has the standard one it came with only.
Our coach has 4 batteries already - we didn't add any to it so I don't know if it was standard or what for the 2015. Our battery compartment doesn't appear to have much additional space for any more batteries either. We've had the fridge running only on battery power for up to about 18 hours, without draining it (I think it was at half when we got back to it)
Can't comment on the in-motion, although I'll be happy to draft off your research!!!
We have the home theater and it's "not bad" in my opinion. It's not great either, a little muffled and we probably will replace it someday, but we are satisfied. We do listen to CD's and our ipods A LOT. I will say that when the coach is moving, it's very hard to hear the stereo in the cab area without really cranking the volume - the speakers are in the living area only (except the bass box, which is in the passenger side corner) The stereo does have an audio in line for the ipod (must buy a cord though)
I would ask for them to install a surge proctector (ours is progressive industries) and ran us about $450 installed. Protects from high/low voltage and miswired campgrounds.
Also have them relocate the trailer hitch harness (that holds the plug for the tow) to the top of the hitch - we have heard they get knocked off pretty often when going over a low area. We had this done at the dealer as well.
Ask for the interior lights to be replaced with LED's (the 2015's are standard with them) and LED awning lights (standard in the 2015 also)
If you have Lippert steps, see if they will swap for the Kwikee, there have been lots of problems with the Lipperts (ours came with Kwikee)
Ours has a pull-out drawer in the basement area, I really like it but some people think it eats up too much storage space. That's a possibility
We've also done a few small modifications ourselves, like adding a bar on the screen door, and a screen door opener latch. Added some command adhesive hooks and other things to make our lives a bit easier (some hooks that go over the cabinet doors to hang kitchen towels, for instance) or a toliet paper holder (got a nice stand at Bed Bath and Beyond)
Good luck on your test drive, these forums are the BEST for information!!!