Mare Island would definitely get my vote, since we live in Napa and are only about 20 minutes away. Napa has an RV campsite (probably availabe for tents - I've never paid attention) at the local Napa Expo Fairgrounds (rather plain but well maintained with a lot of grass & some trees), and there is also Boethe State Park north of Napa between St. Helena & Calistoga - drive time from Boethe thru the Napa Valley to Mare Island would be about 50-60 minutes, longer if there's a lot of commuter traffic (until about 6 p.m.). Could research other lodging in other areas of & you requested, could make a list of various hotels, motels, B&B's, vacation rentals, and tour ideas & prices.
If people were interested in wine tours in the Napa Valley, we may be able to facilitate group bookings, and I would recommend getting a group limo or designated drivers, due to the alcohol - accidents are not uncommon. My wife & I could be designated drivers, depending on the size of the vehicles used or perhaps rented. (Our largest vehicle seating is 5.)
For those that fly in, I'd recommend the Sacramento airport, rather than San Francisco or Oakland (unless you're staying in that area or wish to sightsee on the way in or out), only because it's easy access and navigation from Sacto. to Napa is much simpler & less hectic. Additionally, because Sacto airport isn't massive like the other two, parking is almost directly across from the terminal and 1-3 min. walk - and I've seen free shuttles to/from lots for those with heavy luggage. If someone is a pilot, the Napa Airport is local.
SoCal or NorCal is a possibility, depending on when it takes place.