Wall thickness

Hi,
I have a 15 ft trailer on which I'm planning to build a plywood/pmf camper. The idea is top have a hatch at the back so I can slide in a 14 ft open canoe for transport.
It needs to sleep 4 too so it has to be a fair size.....the missus is also keen on being able to stand up. That might have to change.I don't really want to build a full size caravan.
We're planning on treating it more like a tent than a camper, we'll dry it afer use each time and keep it under cover so it's out of the rain. So that means no insulation, just the one layer of ply, but the thing I'm not sure about is how thick should that layer be? Preferably as thin as possible esp if I end up building it high enough to stand in.
Would I be able to get a way with 6mm ply? Do you guys also use the external grade ply?
I have a 15 ft trailer on which I'm planning to build a plywood/pmf camper. The idea is top have a hatch at the back so I can slide in a 14 ft open canoe for transport.
It needs to sleep 4 too so it has to be a fair size.....the missus is also keen on being able to stand up. That might have to change.I don't really want to build a full size caravan.
We're planning on treating it more like a tent than a camper, we'll dry it afer use each time and keep it under cover so it's out of the rain. So that means no insulation, just the one layer of ply, but the thing I'm not sure about is how thick should that layer be? Preferably as thin as possible esp if I end up building it high enough to stand in.
Would I be able to get a way with 6mm ply? Do you guys also use the external grade ply?