Covers - it's a personal preference. I found it to be more of a pain than it was worth because I use my RV most every month. In addition, I am in and out of it at home and the cover gets in the way of that (and the dogs to sit in the cab and watch the neighborhood goings on.
Water - if you're using it regularly then draining isn't really necessary because you're going through it. If I do empty my fresh water tank, I just run it through my faucets and dump my grey tank. Again, I use it regularly, so I don't worry if there is some left.
Sanitizing - unlike most here, I use swimming pool chlorine granules to sanitize. I use about 1 tablespoon for my 40gal tank. I prefer it because I don't have to do multiple flushes to get rid of the icky clorox taste. I add the granules, fill the tank, let sit overnight and then run through the faucets until empty. Then I simply fill the tank again and I'm good to go. I do drink and cook from my tank and have had no problems. I do only fill from my home water as I have quality water. I have only sanitized twice - once when I first got it and once after I had been sick and it had sat for several months.
Batteries - maintaining the batteries will be affected more by your converter/charger than whether you hook into 15 or 30 amps.
15 vs. 30 amps - if you plan on using your RV at home and using the AC and other electrical appliances at the same time, you will be better off with the 30 amp. If you won't be using multiple items at once then the 15 amp is probably fine. I would start with the 15 for now and if you trip the breaker then upgrade to the 30 amp.
I use the buckets of damprid. For my 21ft rig, a single bucket lasts about 3 months. I make sure to open cabinets with bottles of liquids as the bottles will sweat and must. I wouldn't run the AC constantly.