I put TREX on my flatbed when I re-decked it. Dont think it would be strong enough for you application though ( vehicle hauling)
A 300lb oak round weight is dispersed over an area usually 10+ times the area of a tire. Unless you have it on its side then its maybe 2x the area. Even stacked 3 high your still not quite what a truck would have point load wise.
You can still get agricultural fence posts treated with CCA. I buy a lot of treated products. My main supplier stopped using MCA a good number of years ago and went to MCQ because they think it will last longer but is not as corrosive as ACQ. My supplier told me that his ACQ sales rep told him that if you needed to get rid of a body a 55 gallon drum of ACQ could do it in a few days and leave no trace. Needless to say ACQ is hard on treatment vessels and machinery. And also why you need ACQ rated fasteners when building with it.
Also the quality of the treatment has a big part to play. I had a client provide the round posts for a job. They came from Michigan and as is normal I broke a couple driving them and there was hardly any treatment. Maybe 1/2" penetration at the most in a 4-5" post. My supplier often gets full penetration in a 4-5" post when I break one of theirs.
The posts I imagine is a function of wood and its density. Out west their treated wood is what's the term...incised?? Where they take like little spikes and poke holes or like knife slits in the wood to aid in penetration.
It was red spruce and the posts where as light as bass wood compared to the virginia pine we get here.
Oh yeah, MWP cranks out the product. Their sales team sucks to work with though. They know they are the big boy on the block and if you aren't buying a tractor trailer load a week they will seemingly price materials on a whim. I know a guy who supplies a large amount of raw product to them and he says they are demanding from his side as well, but you have to work with them. I buy almost exclusively from VTS in Zion Crossroads.
100% agreed. Scares the chit out of me when people get on mine and it's wet. When I have to re-deck, that's what will be used.