Ive just been over for the Pre TT Classic and stayed on the campsite at Sulby Claddaghs. Its free, theres toilets, hot and cold water and even a security bloke (Securicor)so I wouldnt think theres too much need to worry if youre young and on your own. Its quite a big site so I would think there'd always be room for an extra tent, and theres a bus to Sulby from Douglas, as far as I know. Sorry, didnt notice any showers there though.