Hi Dalia, welcome to the site.
There are no services at Dumont. Browse the INFO tab at the top of the site and you will find some helpful info about Dumont. http://www.dumontduneriders.com/services/
The closest water is Baker. I would just fill up at home and find a dump station after you leave. Either Tecopa or Baker has them.
Next, Halloween Weekend isn't really an event per say. It's just a busy holiday weekend where everyone goes out and rides the dunes, camps, etc. Comp Hill gets busy at night. It's worth checking out. People used to race up the hill, but now it's pretty much a party with LED lights and people drinking.
Make sure you read up on the rules out there and keep your head on a swivel. It can get chaotic at times, but it's not that bad.