Winnebago Australia had nothing to do with your Motorhome - Knott Industries (Avida) would have built it - and used Winnebago name (A lot of litigation and High Court Orders finally stopped them from doing so).
Each motor home is built to differing specs utilising differing toilets and other plumbing.
Perhaps Avida/Knott may have kept some records on your individual MH - don't hold your breath.
If you get brands and serial numbers of individual toilet cassette and water heater you should be able to identify capacities with a simple on line search.
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Possum; AKA:- Ali El-Aziz Mohamed Gundawiathan
Sent from my imperial66 typewriter using carrier pigeon, message sticks and smoke signals.
