There are a number of places you can empty caravan toilets, there is one here in Howlong just near the footy oval, Never having had a caravan or portaloo I have never had to use one of these things but I assume if you can empty a caravan into it you can do the same with the portaloo.
As for water, I would carry my own hose and just ask at a servo... Couple of years ago with water restrictions servos were not happy to let you fill up but I don't think it is an issue now...
Dave