I just use the 20L water container (one or two) under a silver tarp. This works really well in ensuring that you don't get water pooling.
This. Ours lives outside through NQLD summers, and we've never had an issue with mould or anything. How we park ours:
* The 2 x 20L jerries (empty, and dried out) go on top of the canvas, underneath the PVC cover. They lie down on-side, to give a good gap - depending on how tight your travel cover is, you may need to experiment. We leave the bungie-cords off to get the height we need. This lets the canvas breathe.
* PVC travel cover goes over, and forms a peak down the centre-line of the camper from the jerries. This is all we used to do, but the PCV cover was getting marked from bird poo, native bees, geckos, etc. So I've gone all up-market, and have bought a :
* $4 half a coppers log from Bunnings. Flat side goes down (on top of the jerries, on top of the PCV cover). Round side goes up to protect the:
* Silver tarp from BCF. Bought last time they had a "50% off Tarps" sale. Tarp gets pulled down taught with occy straps to the bottom edge of the trailer. The air-gap between the PVC cover and the tarp let's the PVC breathe and makes sure we don't get mould between the two. Generous overhangs at each end, but open to the air so breeze can blow through.
* Jockey wheel wound all the way up. Otherwise water gets inside the drawbar and just sits. This way it runs straight out. Looks a bit funny, but works beautifully.
And that's it. Make sure the canvas is dry before you pack it away, make sure the insides of the trailer tub and toolbox are dry, and then just leave it as-is. I try and open it all up on fine days, to get a bit of fresh air into the trailer tub and the toolbox. I don't ever open up the tent or unzip the PVC though.
Good luck!
Matto