You are basically limited to Napa Valley Expo in the south part of the valley, and Calistoga RV Park in the north. Napa is nicer, (concrete pads, grassy sites) but in the bigger city. It's near the terminal for the Wine Train if you plan on doing that attraction. Calistoga's downtown spas and restaurants and the Old Faithful geyser are close to the park there. Neither of them is anything close to a "resort", just basic parks adjacent to local fairgrounds to use as home base while exploring the wine country.
Bob - Folsom CA
2008 Fleetwood Southwind 35A, Workhorse W-22 8.1
Acme EZE Tow Dolly - 2007 Honda Odyssey