I went to Spa GP last year and as Tim said in an earlier comment, stayed at a campsite in Francorchamps – entrance opposite the Francorchamps Racing Hotel. (I think it was Campsite 35, but might be allocated a different number this year). We turned up on spec on Thursday and got onto the site no problems. It’s about 5/10 mins walk to the circuit (La Source entrance), near to the bars, shop and steak house with fairly good sized pitches, parking next to your tent, and an excellent purpose built toilet/shower block – you do have to pay for showers but that’s pretty standard, and I don’t mind paying for decent facilities.
Be warned that it does get quite noisy though, plenty of loud music and fireworks into the night, but the party atmosphere is great. Have fun!