Thanks for the question, and it’s a great one to be asking at this stage in the challenge. Your assumptions so far are pretty spot on, and I’ll add some further context to these, as well as point you in the direction of a few existing resources in the design brief.
Vehicle sizes: You can safely assume that the overwhelming majority of vehicles will be 4wd’s, and mostly Toyota (Troopies, Landcruiser, HiLUX 4wd). As far as lift kits go, whilst some vehicles may have been modified, it is best to assume that vehicles are factory standard and so don’t have extra clearance.
Antenna’s/roof awnings/roof racks: All of these are likely to be present on vehicles that are present on the Peninsula. As for caravans, given the rugged terrain and the roads, it’s more likely that there would be 4wd compatible camper trailers if there was anything at all.
Heavy-rigid/freight trucks: These do operate on the Peninsula, with a number of businesses running road trains, including this one which has a run from Cairns to Arukun/Weipa. They also list the various stops they make on that trip, and that could inform where you locate your stations. – https://www.hawkinsqld.com.au
For further reference, head to the gallery on the Challenge website for the Transport and Access section where you will find a number of photos of existing wash stations, as well as a few images of heavy rigid vehicles that can be expected on the Peninsula.