I would suggest Seaforth north of Mackay. It's cheap and a lovely spot. Not sure on the prices now but you park anywhere you like and they have toilets and coin hot showers. Council bloke comes every morning to collect but it's very cheap.
Carmilla is another very nice spot. Right on the beach, No toilets or anything though but free.
St Lawrence there use to be a free stop in the town next to the pool. There's toilets and hot showers by donation. Free as well.
Rockhampton i would stay at the Southside holiday park. We found that one the best.
They are some my suggestions
Have a good trip