My tires on the older pull behind were slow leaking from the side walls due to age. I have plugs here at home but obviously they would not fix my old tires. I did watch a few videos of guys filling tires with cans of foam, but determined that would not last long term as the foam eventually loses its shape and squashes down. Tubes have proven to be the best cost effective fix for me on these old pull behind non driven tires.

there are a bunch of videos of dudes doing their own expanding foam using great stuff and the like. I assume a closed cell foam would be ideal, and most of the other stuff isn't. I assume the tires that you buy with foam are probably also not closed cell. He'll, I'd even wondered about using the "tubeless" tire goop for bikes, except that it gets hard and stops working. For now, the tubes are OK. Getting the tires off of the rims and swapping tubes in/out is a huge pain in the rear, presumably partially because these tires are supposed to be tubeless.