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You might be able tuck the tank up under the expanded metal that you walk on when you connect a trailer to your tractor. I call it a catwalk. I am sure there is some official name for it.
If you put it on the passenger side of the truck, you won't stumble over the bolt heads. Suggest using threaded rod. to affix the tank and maybe a metal plate so the bolt won't slip through...
You just want to be high and tight somewhere. If you spot a better place like behind the step - that could work too. Just don't be stingy with the zip ties to hold the nylon lines close - so that don't get caught in a branch on the ground or something goofy. Be sure to get you some good DOT Approved line to tie the tanks together. Bigger is better for volume.
I am not certain, but a lot of those multiple horn configurations use the one stream of air to power all of them. I have a 5 trumpet set that will rattle your brain - but the 36 inch single trumpet that I have seems louder. It is a lot deeper sound too. Almost like a fog horn. They are a lot of fun to have for sure!