Our mobile home loan contracts lay out the late fee assessment as follows:

If any installment owing under this Note is overdue for more than 10 days, I will pay a late charge of 5% of the overdue installment. If Collateral for this Note is real property, I agree to pay a late charge of 2% of any installment which is overdue by more than 15 days.

We have a loan that is secured by the mobile home and a mortgage on real property. Should the late fee should be 2% after 15, based on the real property collateral?

This is a state law issue, but based on how your agreement reads I would agree your late charge is subject to the 2% and 15 days since it is secured by real property.