I would think secure, away from sparks, where it can’t move or be struck by something that would cause a leak would be good.
I’d feel better mounted fully enclosed somewhere. Easy to put in a belly box since you are in a straight truck.
And regardless if legal or not, it may cause concern if running through tunnels.
The ticket issue may have to do with regs pertaining to fuel tanks, not necessarily a portable tank. That’d be a concern if you attempted to hang the tank outside.
On stuff like this, I’ll bite the bullet and pay a reputable shop to install it.
Mainly to share/pass off liability if needed.