Commodore will HAVE to stay on the mainland, so if you can't squeeze them in, they can hire a 4wd or go on a guided tour. Or walk! You will need at least 3 - 4 days bare minimum to get a brief look at some of the attractions on the island. Lake MacKenzie, Eli Creek, Central station, Maheno wreck and Champagne Pools are just a few of the beautiful spots that you shouldn't miss. Fuel is subsantially more expensive, so plan your trips to get the most in, as there are some great distances to be travelled. More fuel will be used when going through rough terrain or soft sand.
We have always camped at Dundubara, which is a fair way up the island, about 40kms from the barge drop off on the southern end of the island (Inskip end). I can't really comment on the other camping areas, but all the national park ones are fairly similar, and there is also the beach. Cathedral Beach is private ground and I have heard good things about it. There are a couple of barges at the Inskip end, they tend to compete so these are a bit cheaper than those at River Heads or Hervey Bay. Plus if you are coming from down south it is the most direct route, and you also get to go to Rainbow Beach, another nice spot. Another issue is there has been a few track closures on Fraser, so for up to date info check out http://www.epa.qld.gov.au
and go to the parks and forests link. It is a beautiful spot, and is VERY popular, so try not to go on a long weekend or school holidays. Backpackers are everywhere most of the time, so be aware of their potential lack of skill navigating a toyota troupy with 11 backpackers on board, and all the gear on the roof!
Here's a more direct link to epa info http://www.epa.qld.gov.au/projects/park ... i?parkid=1