Best you may be able to do is compare similar sized MH and site built homes in your market--everything as equal as possible--site size, utils, location, etc.
See what the percentage difference is between them. This gives you a MH/site built adjustment factor.
If there TRULY are no MH sales, you will have to use site built as comps, with the adjustment factor you derived above. It won't be a pretty appraisal, but neither are geodesic and berm homes with no comps either, but someone has to do them. Hopefully you can secure the fee you will earn on this one.
1) Make sure you let client know your direction
2) It is OK to far back in time or far away in distance if needed, to get at least ONE MH sale. It doesn't have to be new, in my market, they do not appreciate hardly at all the first few years--can even be worth more as folks add porches, landscaping, paved driveways, etc, as money permits.