Using Thousand Trails with a few stays at RPI campgrounds plus several COEs we averaged around $260/month ($8-$9 a night) over 6 months on the road. When the cost of membership itself is included that rises to around $12 a night. Obviously, it's a good deal for us - probably the difference between being able to move more often rather than do a series of one month stays to catch the monthly rates which are generally quite good.
Still, there are a couple of important considerations:
1. Maybe money isn't that big of an issue and you'd rather stay in higher end campgrounds. If so, Thousand Trails and RPI parks probably aren't for you.
2. Either way, if you don't intend on using the memberships enough to get your money out of them, obviously, you shouldn't become a member. With TT I'd say the breakover point is probably around 50 nights a year.
If you are interested,
I've blogged our camping expenses here.