Not RVing, but, I volunteer with two separate Habitat groups. Here in the northeast they are organized by county. Groups seem to have little interaction with each other.
The groups usually have a website. (Usually not a very good one.) Look for a 'volunteer ' button somewhere on the pages. Ours use a web app called VolunteerUp. Create an account (for each group) and signup for a 'build' day. They'll follow up with an email telling you where to go, who to see and what work is planned that day. It's a very loose/self organizing team.