1. Is it ok to store with gas in tank with a gas stabilizer? If yes, does it matter how much gas?
yes, I put stabil in at last fuel stop prior to arriving home, run generator for at least 30 mins (even while on road) with AC on. stabil says good for up to 6 months.
2. Should it be kept plugged in to the 30amp electrical outlet?, your call, winnebago says NO in their manual to keep from overcharging coach battery (s). I put maint free ones in mine and keep it plugged in. You could disconnect and put a maintainer on them. I use 3amp maintainer on mine sometimes and always on my diesel truck batterys.
3. Should batteries be disconnected? (with or without electrical supply), see #2, I do put a maintainer on the chassis battery, leave connected.
4. Should refrigerator be stored on? (Therefore needing electrical supply).
Its your call no need to, just turn on a couple of days prior to use. Put box of baking soda in refer and freezer to keep smell down. Or you can put some ground coffee in a sock and put them in instead.
5. Should air conditioner be left on? (electrical supply)
Again your call, maybe if in hot humid area such as the south. If dry and hot 5 gal bucket of water is recommended we used to put 2 in when stored outside (have storage shed now). Need to monitor and refill about 1x a month. Supposed to keep wood work from warping/drying out.
6. Is it important for it to be stored level? If yes, tires only or leveling jacks too?
Issue if refer is left on and out of level. I leave on tires with plastic bed liner material under them. But mine is on concrete, but even if on dirt it would be beneficial. Also cover them. Check your manual on leaving the levelers down. Some say yes others no due to possible contaminants becoming lodged on the shafts.
Again, thank you!!!