We caravan with our sheltie, collie sheltie cross and our rottweiler. Also have 3 children and daughters boyfriend, so it can get a bit tight for space sometimes in our 6 berth. The dogs have crates in the awning and shelter outside during the day but sleep in the caravan unless it's too hot.
Some sites say that they only take 2 dogs but its worth giving them a call as many will work on individual basis. many sites say 2 so that they have a clause if your dogs cause a nuisance. Sites usually charge by the dog but if they are genuinely dog friendly sites they may be open to offers. All the sites that we use regularly, allow us to pay for 2 dogs. Recently found a site in Cornwall near Bude (haven't been there yet) that have no maximum on numbers of dogs and no charge either, but have the usual behaviour rules. We are off to Dawlish in 3 weeks to a site that has no number restrictions on dogs.
When we visit a new site, if we like it enough to plan to return we always look for boarding kennels in the area so we can check them personally. That way when we return we know we can trust the kennels if we want a day or two without the dogs eg to go to an attraction that wont allow dogs or to enjoy a night out whilst caravanning. All sites say that you must not leave a dog unattended so a kennels that you can trust is a good option.
There are quite a few dog friendly websites check these 2 which give dog friendly campsites.
www.innerwolf.co.uk/gbu0-display/Dog_Friendly_Campsites.html
www.dogfriendlybritain.co.uk/directory.asp?Category=16
or why not ask on here for personal reccomendations in specific areas.
We use dog bags in the car which double as 'tents' for the dogs giving shelter from rain and/or sun. We have also recently found a windbreak product that allows you to create dog pens www.dandt.eu/