In a case where the bank did a loan to purchase a primary dwelling and secured by that dwelling and the lender also took the existing primary dwelling but the existing dwelling consisted of land and a mobile home and only the land was taken as collateral, would that be subject to rescission?

ROR does not apply to purchase loans.
The property taken as AOC doesn't change that.

If the property taken as AOC is the consumer's current primary dwelling rescission would most definitely apply.

Review the Commentary to 1026.23(a)(1)(4).

If the MH is not part of the real estate and the FI did not take a security interest in the MH then rescission would not apply. But if the MH is considered part of the land then it would be secured by the mortgage and if the MH is the borrower's primary dwelling rescission would apply.

We took the primary residence (1-4 family) being purchased and just the land of the prior principal residence so I'm in agreement that ROR would not apply here.

As long as you are sure the MH is not part of the land then I would agree. But I would advise to determine if the MH is taxed as part of the land or if it is still taxed as personal property (titled property).

If it is part of the property it secures the loan by virtue of the mortgage.

Also, in regard to assuming ROR doesn't apply to Purchase transactions, bear in mind this would also not be the case if the purchase or any of the purchase money was placed on a HELOC.