Hi Adam, and welcome aboard.
I'll be on Fraser from the 19th Dec so keep an eye out for me - I might even buy you a beer
All good advice above, just remember to keep an eye on the tides. All beach driving should be done within 2 hours either side of low tide, but there's lot's of inland roads.
The EPA website gives fortnightly updates on conditions on the isalnd, so keep an eye on that. Inland tracks can vary from week to week depending on how much rain they've had. At the moment they are very rough.
Generally I've found that fuel on the Island is around $0.40 dearer per litre than Brissy, and your fuel economy will vary from 15l/100 without the trailer to 24l/100 with the trailer.
Have fun