Jon, thats clever and not bad at all provided you have plenty of space. But if you navigate as if you want to break sh#t it really must be strapped down. This particular tank is a high pressure steel tank (= heavy when full). Flying about it would put a hole straight through the hull; regardless of construction material/quality (or a nasty dent at the very least).
Ps. Given that you’re from Oz, do you see many of your mates running full size tires on their trailers? Reason being, when towing your RHIB off road I suspect that having strong tires that are under inflated would allow the tire to absorb / dampen irregularities in addition to the trailer springs/suspension, ergo less potential damage to the RHIB.
Bogmonster on here has a very nice trailer setup for off road; as do a few other members whose names escape me now, but please post off road RHIB trailers from around Oz if you happen to come across any.