If you're getting a generator, you might not need another battery. A second battery is nice if you're going w/o a generator or relying on solar, since you might end up under tree cover. You can also saving electricity in other ways such as watching how much you use your lights. In July, the days are long so that shouldn't be a problem.
We were there the second week of July last year and never needed heat. We have a decent blanket and comforter that keeps us plenty warm that time of year. I have found that since I am follically challenged (thinning hair) a stocking cap is sometimes handy in the middle of the night.
As for water, we used the showers at Grant Village. This saves needing to fill your water tank and worrying about your gray water tank. There are also showers at Canyon. IIFC, they are $7.00 +/- per person, and you can take as long as you need. :) Two free showers per day are included with your reservation at Grant or Canyon. We also use the park toilets rather than our own. So all we are putting down the drain is dish water and some water when we wash up or brush our teeth, etc. We stayed a week at Grant last summer and didn't come close to half filling our gray water tank.