Hello,
Since the rig is more than 20 years old, you're obviously not interested in full insurance (collision and comp), so all you'll need is liability insurance. I'd start with your current insurance company, they usually offer multi-vehicle discounts. Also, ask about a discount for only using the vehicle only part of the year if you're not going to camp year round.
As far as price, there's just too many factors to consider. It varies by state, county, driver history, credit rating, etc etc etc.
Anyway, sound advice offered regarding tires. Having a blowout is no fun, so make sure those tires are good and not too old.
Also, if you're camping with that many people, check with the campground before going. Lots of them charge for more than 2 adults and 2 kids.
Have fun on you new adventures!