The trailer tail aerodynamic devices are supposed to save 5% on fuel. They remain rare and 99% of those are left undeployed. The trucking companies don't seem to care much about easy cost savings.
Especially for local deliveries- the truck is often half-empty.
Still even at interstate speeds a 5% savings pays for itself pretty quickly.
The fuel tank of a semi truck is not an important proportion of the weight of the whole vehicle, so it does matter, but not so much.
They may not be worth it for short haul trucks, but long haul trucks I bet the savings adds up quick.