Either your loan is paying off an existing loan secured by the dirt (in which case your TRID purpose is a refinance) or there are no existing loans secured by the dirt and they are pledging the land they own free and clear as collateral for this loan (in which case, your TRID purpose would be home equity).
Unless somehow they are buying the land with the mobile home (in which case you'd have a TRID purchase)--that's not how i took your original post.
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I'm fixin' to fix that.