We went from Cairns to Brisbane. Took us 3 1/2 weeks travelling with 2 small kids. I'm sure could be done quicker but with kids 6 hours driving per day was our max. Make sure you head off the coastal road. Atherton Tablelands, Waterfall circuit, Cania Gorge, Eungella NP to name but a few.
Snorkelling north of Cairns will be best AFAIK. Just be careful driving in Eungella NP as serious steep inclines and campervan may not be up to the job.I was sure we were goners a few times...kids loved it of course!
If your camper fridge only works off elec. Make sure you dont do your shopping until you know you can plug it in somewhere.
Buy a cheap fan in one of the big stores it was invaluable to us at $11 especially in Northern Queensland.
Don't plan everyday of trip we gave ourselves a bit of leeway, meant we could leave a place early if we didn't like it..Cairns for us and we stayed few days extra in Cania Gorge camping place beside NP..heaven.
I'm really jealous, but at least I have my memories...Enjoy.