Q1. £7000 seems very reasonable, but depends on condition. 10 years old is not really old, it makes it a 2002 van which I would think is ok.
Q2. Ground rent £1700 per year, depends on the site licence, some holiday sites are licenced for 46 weeks per year, some for only 8 months.
Q3. plus gas and electric is normal, holiday vans are usually bottled gas, and leccy is privately metered.
The price of £7000 can be negotiated, and also your parents can ask for free ground rent for the first year. You can never make money by buying a holiday caravan, it's like a car, they depreciate in value, but there are ways and means of selling when the time comes, also during the time your parents own it they could get some money back by renting out to friends and family when they are not using it. Finally the advantages of being by the sea in their own place and the pleasure they will get can't be put into money. Anyway best of luck with their decision.