I would certainly contact the WI DNR as suggested above. The ranger was not running a scam but the front office screwed up. It happens. But I can tell you being a WI resident this is not the norm. We've camped in many of the state parks, and they always ask if we have a sticker or if we need one.
If you stayed at Lake Wissota State park you should have been changed a daily vehicle fee. Again, might be worth mentioning when you call the DNR.
Miller RV park in Dubuque is in Iowa, not WI. Different state, different rules.
