I am going to assume you are referring to the HMDA rules that go into effect on 1/1/18.
If you have a transaction secured by a manufactured home community you do have a dwelling. Even if you are taking the manufactured home community but no individual units, it is still considered to be a dwelling under the definition of dwelling (1003.2(f)-2).
There is a particular data field that requires you to report the number of dwelling units. In that field you would report the number of sites that secure the loan and are available for occupancy regardless of whether the sites are currently occupied or have manufactured homes sitting on them. So if a 20 pad manufactured home park is taken as collateral, you would report all 20 units even if only 10 are occupied with a manufactured home on the site (1003.4(a)(31)-2).