It is FR123, Battleground Ridge, and it is accessible with 2wd high clearance. Sedan possible if you're careful. I wouldn't think it would be any problem getting a popup out there.
On the point by the pump station it is really rocky and unpleasant for camping, but a little bit south of there you will find better spots. The side road to the Battle of Big Dry Wash Historical Marker also leads to relatively good camping, though that road is a bit rougher.
There is a hiking trail that begins at the pumping station and leads down to the lake at a very popular boat-camping spot. It's a short hike and not too steep. I've seen people carry their cooler down there, but that is a serious chore in my opinion.
Please note that under the new travel management rules, you are not permitted to take your vehicle or popup more than 30-feet off this portion of FR123 for camping. Basically, you can only park your vehicle on the side of the road and camp there. Though you will likely see previous side roads and camp sites that have been used in the past, it is no longer permitted to do so in this area. (The southern portion of FR123, nearest to FR300, allows dispersed camping up to 300-feet from the road, but not the northern half closest to the reservoir).
Maps and travel rules are available here:
http://www.fs.usda.gov/detail/coconino/ ... rdb5356224