I think if you look through the old threads there's quite a lot of discussion about Eau Rouge camping.
It's a nice "normal" campsite (by that I mean a site that is used by families all the year round), it has good'ish facilities and a bar - they speak OK English and you can book over t'internet. We have stayed there for a number of years for the 24 hour and would recommend it.
It is a trek to the circuit, we have a media pass for the 24 so we drive but Rick, Bobblehat et al reckoned if you are walking it's an hour from the campsite to the bottom of Eau Rouge at the circuit. You have to walk up the road to the rear entrance (Karting entrance) of the circuit and then up the tarmac road through the woods. It's obviously quicker if you take bikes with you.
I'm told that the traffic is pretty bad for the GP so I'd probably go for camping and walking/cycling to the circuit rather than trying to stay much further away. There isn't a vast amount of accommodation within a couple of miles to the circuit so you'll end up 5/10 miles away at a minimum (probably further unless you book a package) and it might take you some time to get to the circuit by road.
I'd try to get there for Friday, the Friday of a GP weekend has lots of tracktime (because of the test cars - assuming that's the same this year?) plus it's always a quieter day so you get to walk around the circuit and it should be law that everyone has to walk as far as possible around Spa
PS - don't forget to eat frites