Went there last August and had a great holiday with wife and 6 year-old in my Vanagon and my mate (with wife and 6 year-old) camped in the next pitch in his T4 Westy. We stayed at Fauxquets Campsite (
http://www.fauxquets.co.uk" onclick="window.open(this.href);return false;) which was excellent, particularly for families - cycling distance to the beach, walking distance to a couple of pubs. On-site basic restaurant.
Guernsey itself is also an excellent place to visit - I call it Cornwall without the crowds and traffic jams. I suspect the cost of the ferry puts people off and they stay in Dorset/Devon instead. Up until last year Campervans were not encouraged on Guernsey and just to take one required a £40 permit and you were still not officially allowed to sleep in it (even on a campsite)! Things are now much easier: permits are issued free providing you book with a campsite licensed to take campervans (Fauxquets have a complete row set aside for campers, so you should be next to other campervan types!). Vehicles have to be less than 6 meters long (no motorhomes).
Last point, May should be OK but still worth remembering that Weymouth is hosting the Olympic Games sailing this summer and I suspect traffic in and out of the town will be a nightmare. Slow ferry runs out of Portsmouth.
Have a good holiday.