Maybe FMCA rather than "dilute" their MH base, should consider adding a sister club, say something like "FTCA, Family Trailer Coach Association". The two groups could be "blended" for some rallys, and some not. Make many of the benefits cross over, tires, roadside assistance, heath care, ect.
Push the point that the club is all about RVing, in whatever you do it in. The life blood of the FMCA is aging out, and there is a strong need to get more younger family's involved. It is a testimony that it is not uncommon for us to visit a campground, especially a state park or COE campground and discover that we are the only MH among 50 towables.
Winnebago had an issue like this come up about 5 or so years ago when their own club, WIT - Winnebago/Itasca Travelers decided to allow towables. Winnebago had purchased Sunnybrook RV and started making and branding Winnebago towables. Now any Winnebago or Sunnybrook towable is welcomed at the "GNR- Grand National Rally" in Forest City each year. They made the jump and lived through it.