I have towed camper into Wonnangatta valley three times and have driven in there many times without a camper. The only way I have gone in is East buffalo, east Riley and then down the Wonnangatta track across the river to the valley floor. There is only one section on the Wonnangatta track that is reasonably steep, but take your time and it is ok. If there is any rain forecast for the area I wouldn't attempt it.
Last time we went in with the camper,we left the valley via the wombat range track and the onto the Cynthia range track, I would not attempt this route with a camper in tow unless you have a high level of off-road towing experience.