You'll just have to start calling around. The above link for RVParkReviews is a good one to give you an idea. However, don't take any reviews (on any site) as gospel. Eliminate the excellent reviews and the worst reviews and concentrate on the middle ground ones. Some things in reviews wouldn't pertain to you like not having a pool, etc.
Once you're there you can drive around to different parks to see if one would suit you better for your next trip. You might also consider renting in two to four different areas for weekly rates - even if it's more costly that way - to give you an idea of the areas and prices. Your first trip should be geared for looking for the best place for youself on subsequent trips. Everyone has different criteria for where they stay.
Also, at this late stage you might have better luck getting a weekly site rather than a monthly. Good luck!
