Just go buy you a couple of plain old air pressure gauges and plumb one in on the trailer and the other on the truck, probably less than $50 total. You can use the scrap yard scales to get you're pressure readings at a given weight and go from there.
Hauling scrap is a crap shoot. When people load scrap trailers, they just keep piling it in until it's full. If it's something bulky, you usually fill up before reaching max weight. If it's something dense and heavy though, they will grossly overload you. I once saw a truck pulling a 40yd scrap hopper go over the scrap yard scales grossing 146,000lb.