There are only two, Equity Lifestyle Properties (ELS) and Sun Communities (SUI). Both focus on lifestyle communities in the sunbelt and are around 50/50 MH/RV. ELS has the higher quality portfolio but has a lower dividend than SUI. There's also UMH, but they are a non-traded REIT.
Other publicly-traded investment companies invest in MHPs, but are not REITs, and are not just MHP investors. Some examples are Tricon (TSE, although I hear their portfolio is on the market), and Colony NorthStar (CLNS).
Hope that helps.