I didn't even have to 'horse' the culvert into place in the creek bed, the straight truck they delivered it on had a knuckle boom on board unloading crane and they sat it smack dab in the middle of the creek bed for me no charge. Pre cast comes in different wall thicknesses. I bought the thickest wall they had in stock pus the county was all good with it as they came out and inspected it.
I do like your repurposed trailer bed idea however. My concern would be how slippery it would be when wet or had ice on it. Aluminum decks are inherently slippery when wet and I'd be worried about the under frame being cracked as well. I spent a lot of my working years repairing cracked frames on aluminum trailers. Seems as though they a crack over time.