> The real benefit I think is when stuff like tax codes, carbon credits and government subsidies come into play and ROI goes down significantly.

Those are unreliable and vary at the whim of the government and their need to fill a hole in the budget. The only reliable factor here is a subsidy on the purchase price for the trailer as that can not be withdrawn after a year. Carbon credits will disappear as soon as a significant part of transport has been electrified, electricity prices are still rising, tax exemptions are temporary at best.