My concern is this will take the emphasis off motorhomes, and motorhome specific issues, and turn it into another Good Sam's style RV club,
I think that is a given if it is opened up to the entire RVing community. Whether it would be inferior or superior remains to be determined, but there is a fundamental difference in that FMCA is a non-profit owned by its members while GS is a for-profit business operated for the benefit of its stockholders rather than for RVers.
For FMCA, it boils down to survival. They cannot afford the infrastructure and services they want to provide with the declining membership. If they have to cut services, they lose even more members. They have not been successful attracting younger MH buyers, so hoping to do better with towable owners. Widening the market is bound to help some, but may not be sufficient for the need. If many MH owners don't feel the need for FMCA services & benefits, why will towable owners feel any different? Especially since their demographics are much like the MH owners who do NOT choose FMCA (younger, family-oriented, etc).
FMCA has also suffered a major financial setback with the declining revenue from their excellent magazine. It used to provide a strong revenue stream that was used to fund extra services, but advertising is way down as buyers flock to the internet for info and the manufacturers concentrate their ad budget on interactive media. FMCA's print magazine is probably doomed, just like most other print media.
Since we no longer own an RV or belong to FMCA, our dog isn't in this fight. In my opinion, though, adding towables won't really solve any problem for FMCA. However, it might be good for RVing to have an honest competitor for GS, and RV owners surely could use a voice that speaks for them rather than for improved profits in the RV industry. The latter, however, means that FMCA would have to become much more proactive in RV affairs, a role they have previously shunned.