Roadmaster went bankrupt right along with Monaco. Part of the same bankruptcy filing, since Monaco was the majority shareholder of Roadmaster as well as being its only customer. So when Monaco went under, Roadmaster went with it. So with neither the original Roadmaster or Monaco in existence, there is nobody left to assume liability for any problems, whether warranty or safety recall.
Technically any owner of those chassis can file a warranty claim against the defunct company, but since there are no assets and there is already a long list in claimants who received little or nothing in compensation, it is a waste of time and money to do so. The bankruptcy court will just add the claimants to the list of people who get nothing.
The new Monaco LLC subsidiary of Navistar bought the assets of the old Monaco company, including the Roadmaster chassis designs and manufacturing equipment and the right to use the names. But they got none of the liabilities.