There's a dump station with potable water at the campground, you'll see it on the left just after you pay at the entrance station. You don't have to camp on the lake in the sand, there is another area above the lake next to the dump station as well.
You will be on the lake. Don't know about potable...but there is a paid state campground probably a mile away...look at their website..Sheps Canyon or Angostura State Rec Area. BEWARE lots of ticks n biting flies. I pulled over 30 off dog...and flies still in van!