I'd always imagined that the Collins family was English and wealthy, and that a second son (Isaac?), seeing that his prospects were limited (since the oldest son would inherit), decided to have a go of it in the colonies, maybe with a royal land grant. But of course, I have an active imagination.
The 1991 revival series idea about the house being brought over brick by brick would have been believable had the house been a little smaller. And that idea might indicate that perhaps Dan Curtis imagined (reimagined?) the Collinses as a wealthy English family.
Then again who knows? Maybe they were from Cleveland.