Sounds like a plan, you could continue through to Toowoomba, Crows Nest, Yarraman and head to Goomeri then Kilkivan to join Bruce Highway north of Gympie then up to Maryborough and the Bay.
Some hills(up and down) whichever way you go as you are on the Great Dividing Range.
I would just head up the New England to Warwick and then decide to go via Brisbane or Toowoomba.
It would depend on the time of day you would pass through Brisbane. If you get to the Southern outskirts of Brisbane around 10 am you will be past rush hour and able to drive on the Gateway and head North up the Bruce Highway to Maryborough then Hervey Bay. Same in the afternoon if you get to the Southern side of Brisbane from 2pm on you will hit the rush hour traffic heading North.
If it's either of those scenarios I would go the Toowoomba route. Longer and a few hills and windy bits but nothing too severe. Not too much in it re time.
If you're heading to Toowoomba, the camp in Nobby is popular (in that there always seems to be people there). It's in the main street and across from the pub if wanting a meal. After you pass through Toowoomba there's Hampton. Again generally people there and further along there is Cooyar. We've camped at Cooyar. Big area by the Swinging walk bridge. Turn off the highway on the right, go past the pub. No facilities but there is toilets in the park across from the pub. If going down the range from Toowoomba to Brisbane there's a large area at Helidon on the left just before you go over the bridge and the highway becomes the Gatton Bypass. Otherwise you can go through Gatton and they have a free camp right in town and you can walk up to the shops if needed. No facilities I don't think. It was put in place last year if I remember correctly. If you need to stop before getting to Harvey Bay there is an area in Maryborough. We've stayed in that one and again you can walk to the shops and the information centre. It's bitumen and there is an area for Motorhomes and a couple places for longer rigs. You must get a permit from the information centre and display it as it is policed, but it's free. Across the road there is free toilets and showers even if you just need to stop and freshen up. Others will need to tell you about places in NSW as we've not free camped along the New England Highway. Cheers.
