Damn man! Are you seriously considering taking a bus from jo'burg? I tell you one thing, consider your self lucky if you make it through the bus ride alive...I'm mean with all the big luggage, the chances are near zero!!!!!!! Google "INTERCAPE"....
WHAT BS!!, you clearly dont know WTF you are talking about.
Many people has taken the bus from Joburg to Maun. I must agree that it's not the best way, it will be hot, slow and will take at least 24 - 36 hours to get to Maun.
My opinion just bite the 'bullet' and get onto an Air Botswana flight from Johannesburg to Maun. Safest, fastest and best option of travel! You also stand less of a chance of belongings going missing while you traveling.
See you chaps soon!