There are a few campgrounds open into October - however, many of them close closer to the beginning of the month. I've got a list here of all the tentitive closing dates in October. Lord Selkirk and Seal Cove will be right on the water, lovely areas outside of larger communities. New Glasgow Highlands has some really lovely treed sites and is located inland.
As for weather, it varies from year to year. Some years we've had beautiful warm weather where I was wearing shorts to school every day. Other years it's been so dreadful cold I wouldn't have ever dreamt of camping. I'd keep an eye on the weather and have a back up plan just in case!
Vistabay Chalets, Golf, & RV Park - Oct 15th
Cornwall KOA - Oct 12th
Points East -
Lord Selkirk Campground - Oct 12th
Seal Cove Campground - Oct 4th
River RV Campground - Oct 11th
Green Gables Shore -
Cavendish KOA - Oct 4th
Marco Polo Land Campground - Oct 15th
New Glasgow Highlands Campground - Oct 31st
There are companies that arrange trailer rentals, but they are closed by the end of September. To my knowledge, there are no RV rentals available on Prince Edward Island.
Hope this helps with what you decide to do. Feel free to contact us before you come for any additional details or updates on campground closing dates. You can always call us at 1-800-463-4734! Thank you, too, for choosing Prince Edward Island for your destination celebration, and happy tenth anniversary to both of you!