You have to refer to state law. If the MH is on a foundation with the wheels and tongue removed then under state law it could very well now be considered real property instead of personal property. In that circumstance then your mortgage not only covers the land but the MH also and in most instances you would not be able to perfect a security interest in the MH with a security interest in the land it sits on. And all "real estate" regulations would kick in. Including the NHLA and RESPA.
When we do MH loans we require a copy of the tax assessment which will indicate whether the MH is being taxed as real or personal property and we then will perfect our security interest as needed.
As I said earlier, the NHLA is not required on a MH only secured loan, however as Lucy referenced you have to understand your state law. And if in doubt take Lucy's advice and treat MH loans as being covered. This is one instance where there is no violation for providing the notice when it is not required.
The opinions expressed are mine and they are not to be taken as legal advice.