As described before the ruins are gorgeous and well worth the trip. In my opinion you need at least three hours to check them all out.
We first thought we could camp at the parking lot of the ruins but that is not allowed. So we had to drive back 40km to the parking lot of the museum. That takes about 1.5h and you do not really want to do that in the dark because of the low hanging branches.
At the parking lot of the museum the guards wanted additional 64$ pp for camping, after some negotiation we paid 50$ in total for two people.
The parking lot is - as described - nothing fancy but leveled and quiet during the night except jungle sounds.
Place: Parking lot Cakamul museum
Location: 18.36544, -89.89261