Is it permissible for me to empty an R.V. holding tank waste into a manhole?

NO. The only people who are authorized to open manholes are the Authority staff and people who are doing engineering or work relative to a sewer. This can be a hazardous activity and should not be attempted by anyone who has not had the proper training. Many people have installed a special clean-out right on their own property for this purpose. Presently there are no local septage dumping facilities, however that may be a consideration at some future point.