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Let's see - uncrowded beaches in August, wilderness in the northeast, all within 10 days round trip of the most congested area in the United States. Hmmm... Believe it or not, I think that may be possible, but only if your RV rental firm allows taking the rig into Canada and you all have passports. If you meet both of those requisites, then you should be looking at northern Maine and the Maritimes of Canada, in particular the Bay of Fundy, Prince Edward Island and Nova Scotia. Otherwise, I'd suggest the Delmarva Peninsula, the Outer Banks of North Carolina, and Cumberland Island, Georgia, none of which will be as uncrowded. Unfortunately, I expect that most of the beaches on the Gulf coast are going to be unusable for a while.
AZBuck