If the traveller site goes ahead it will seriously depress house prices locally. Many people will simply never consider living within 1/4 mile of a camp for fear of thieving, hygiene and noise problems. The fact there a camp is being planned nearby should already be depressing the value of the house. Does the current price reflect this? From what you have written it doesn't appear so, so a 90% offer on the asking ought to get some traction.
It seems to me you are thinking of gambling over half a million pounds on the expectation that the traveller camp doesn't get agreed. However, you should be aware that travellers have been known to move in anyway and set up camp. Because councils have been slow to comply with the requirement to provide sufficient sites, it becomes far more difficult to evict and the decision over which site to select becomes made for them. Also if the travellers buy the land first it becomes near impossible to evict.
I wouldn't do it, but it's your gamble. Good luck whatever you decide.