Hi, we stayed at Huttopia last year and I agree, it is quite expensive (around £50 per night for 4 then). I also looked at 4 Vents which is around 40 miles from Versailles and the trains from Touran to Paris were more expensive especially as we wanted to spend 2 of our days in the city. From Huttopia we walked to Versailles, about 30 mins, and the station was a 6 min walk from the site - the fare was around 6 Euros return to the Eiffel Tower. We'd stay there again as it was so convenient for what we needed to do, by the time we'd wandered around Paris all day it was nice to just get on the train without having to drive back to the site - we didn't use the car at all.
Just a bit further south is Camping la Beau Village at Villiers sur Orge, this was going to be our second option, I think it was cheaper.
I pre-booked our tickets online for the Eiffel Tower which saved some time but we still queued to get into the lift to the top, I wish we'd done the same with Versailles as we queued for well over an hour there (only 5 ticket desks) - I think you have to go early.....