I have only done the upper Murray (Brigenbrong Bridge to Albury) although I head off again tomorrow from Albury. A little different from the lower Murray where you are thinking of but I thought I chime in.
While it can be subjective, I suspect if you spend a leisurely 6-8 hours on the water that you would probably easily average 35+kms a day. While the flow this year should be better than it has been in some time, I am not sure about how much current this translates to in the lower Murray. You also have to remember that you may get some days where the winds may be against slowing you down. I myself am planning on doing around 35kms a day (when factoring in a rest day about once a week).
As for distances (and other things) a helpful planning website is for QHover (http://murray.qhover.com/
). The website has information on the locks (although you may have to check that the phone numbers are current) as well as page with all the distances along with accompanying photographs taken from many of those points. If you were going to do a five day trip to Mannum, maybe starting from Morgan would be best?
As for camping, I have stuck to designated camp sites so far. I have been using the Murray River Access books from Spatial Vision which have most of them marked. I also have used sites like findacamp.com.au as well. In theory the Victorian side of the river is supposed to be a reserve however it is not really that simple (there are warnings in the MRA books about not camping on licensed water frontages).
As for weight, I am not sure what you are taking but remember that you will probably be passing through towns on your way where you can grab food/water etc so depending on which part of the river you decide to paddle you may be able to save some weight that way. I plan on stopping in towns/caravan parks every few days to refill with water and hopefully buy any items. I have a couple of smaller drom bags and Nalgene bottles to carry water and if I run out I have a Sawyer Mini plus Katadyn tablets for filtering/treating water.
Lastly, I am powering my electronic items using a Anker PowerPort Solar Lite folding solar panel (15W) and also have a 10000mAh USB rechargable battery that I keep topped up in case I run into a few rainy days.
If you can move your trip up to January when I hopefully will reach down there, maybe we can paddle a section of the Lower Murray together? If you are interested PM me ...