I had similar thoughts when I first started planning a trip over there years ago and kinda ran into a dead end. What I found is that most of the gas stations and convenience stores are too busy turning over higher dollar items on a race weekend to be bothered to get into the water business. I'm sure there's a source for water nearby but I never found it. Hauling full tanks of water up over the top of Kilington Mtn in Vermont doesn't appeal to me either. The campground across from Quechee Gorge is an option but it's still a good haul to the track from there so I gave up on the idea. You may want to check out this place if you haven't already purchased a camping pass for the track property: Home - Speedway Sports Park
They are located right next to the track between the South and Main entrances and offer dry camping for up to 500 RV's on their property. Price is $275 and you can come in as early as this weekend and stay as late as Monday after the race (10 days if your're a real diehard). You can fill up your tanks for free as you enter the property. Free dump station and free hot shower facilities as well. Free concert on the property Sat night before the race. Camp store for ice, etc right on site. They are actually closer to the track (by a lot) than many of the speedway camping lots. Track camping prices this year start at $140 for a 20'x30' and go up to $255 for a 20'x60. A dump charge on they way out is at least $25 or more. By the time you add in $25 for water and assorted $6.00 showers, the Speedway Sports Park offers a lot of value and a lot less walking to get to the track. Also, keep in mind that the track has gone to 100% reserved camping for the first time this year, so expect a mess as folks try to shoehorn in between a couple of already reserved spots. Gone are the good old days of showing up and grabbing the next spot in line on the hill. I don't have a vested interest in the Sports Park - I'm just a happy camper there. Call Emily at (603) 783-4088. Tell her Tom Coutant referred you and you'll save $25.00 off the rate. See ya at the races!