I agree with Jo Ann. So, how about very dated sales comparisons? Do you have very dated MH sales that occured some point in time in comparison to similar sized SFRs at the same time? It's a complex assignment but, using dated paired sales along with current SFR sales may help you determine a value adjustment presently.
I've always indicated to my trainees: If an adjustment "fits" in the grid and makes total sense to the bottom line and can not be disproved to any greater extent than it can be proved, how can you be faulted?
The bottom line is, a good appraiser can not manufacture a Comp.