I am not an attorney. It looks to me as if you need a legal opinion about the right of the ROW owner to prevent you from maintaining your road (in dotted lines?). Does the ROW owner have his own road? Does he own property along and/or use your road? If the latter, I believe he would have to be included in a statutory association.
The ROW owner should be able to put up a picnic table if it does not interfere with your ability to use the ROW. Do Town Codes allow for the trailer on the ROW? It appears to me that you do not have rights to maintain the ROW except for the piece of your road that crosses the ROW unless your deeded use is impaired. If erosion of the ROW is affecting the body of water the DEP should be able to help you.
I do not believe you need this owner's permission to form a statutory association. Depending on the answers to my questions in paragraph 1, he may be one of the owners needed to vote a majority in favor of forming an association.
It is best to work out an agreement on your issues of concern with the owner of the ROW. A statutory road association of all owners along the road to share maintenance costs makes sense to me.