Do you really intend doing some dirt road?
Normanton to Bamaga at a glance is dirt
Are you sure? 
I spent hours on Google Maps in satellite view - it all appears to be sealed (even with a white dotted line) along chosen path... I hope. Even Wikipeidia says so:
"The road is sealed for its full length, but as of 2016 there are many sections of the road which are only single lane bitumen." ... the "single lane" bit worries me a little, I must admit.
Looking at your route you aren't going anywhere near Bamaga. The road from Isa to Normanton does have some narrow sections as does the road from Normanton to Cairns. Just watch out for trucks and make sure you get off the road for them, they won't get off the road for you. Caravanners are unpredictable, some will drop a wheel off letting you stay on and others (arseholes) will take the right of way and force you off the edge. Some of the edges have nasty drop offs which makes getting back on again a little tricky and if there has been rain they can get quite soft as well. Good luck with your trip.
Thanks for the advice Bikebear.
Cos I'm a nerd, reviewing the entire Gulf Dev. Road again, using both satellite and streeview (mostly not updated since 2008), all of it is definitely sealed. The majority of it is 2 lane (with solid/dotted line). It's probably safer to assume that the roads with no marking would be narrower than "normal", although some unmarked sections are certainly as wide as others with markings. Anyway, there's a total of about 60km of single lane (no markings) over the 450km of the Gulf Dev Rd with the longest being around 18km. (
Map here). I'm hopeful that it's still under development so I'll contact the local shire to confirm closer to departure.... I'm still nervous about some of the larger sections but hopefully they'll reduce before I leave.
