We have one neighbor with a pontoon boat parked in the grass in their backyard and another one with a camper in theirs.

If it were up to me, they wouldn't be allowed to have them anywhere but in the garage, but it's not up to me and I don't spend a ton of time worrying about it.

If the subdivision every starts putting an HOA together, I'll likely try to push for no parking of boats, campers, cars, etc. in yards.