i have a 79jayco on road & i have taken it onto the sand at straddie & befor that i had a chesney pop up & did the same thing. the only problem i had with the sand was height. under slinging the axel will make a difference also maybe a set of mags for a wider foot print, tyre size will depend on the wheel arches mine had 13's but now 14's just for a little more height. as for going bush the best sugestion i could make is find a good camp spot as a base camp where you dont have to take the camper over the real rough stuff. even some of the jayco's & alike [off road models ] can break.
just my opinion the colman's arnt a bad camper just make sure if it has a fridge get a gas certificate if you live in qld also a roof vent as they can get hot inside in summer.