mchero,
I have a 2005 version of your MH, if they're the same as far as the back end, here are some thoughts for you.
Look at the battery holder. Mine will hold five batteries and there is enough room for a good welder / fabricator to add additional space to the rear of it to hold a 6th battery. That will give you 50% more battery capacity.
If you change batteries, you can also go to the larger / taller Trojan battery. What you have installed should be the equivalent of a Trojan 100 and there's plenty of space over the batteries to go to the Trojan 125 for extra capacity if you don't want to add more batteries.
I believe there is room between the rear wall of the closet and the rear cap of the MH to run the wires down to the batteries. Not that there's anything wrong with going through the closet, but you can keep the wires out of the closet if you want. You might pull out the third brake light, Cyclops light, and run a snake up towards the roof to see if you get to the top. And maybe take out one of the clearance lights and run the snake downward to see if there's anything in the way.
I've thought of solar on mine, but hadn't seen anyone do it yet to a Discovery or Excursion. There's certainly a lot of room up there. And there's so much room, that if you're going to install solar, you have plenty of room to install more than 400W and still leave walking room. I think I could easily get five 120W panels when I was looking at it a couple of years ago.
Bill