Does the environment in each area have an impact on the printing? If you're just rolling them around in a common environment and plugging into AC when you arrive, it doesn't seem useful to me but maybe I'm missing something. Are they different printers (models, label stock, ink, etc.)? If not, I would think you could justify not fully testing each one (in each area) by equivalence. Might be good to take a step back and re-consider the validation plan.
Since it's mobile equipment, I would be more concerned about the impact from the rigors of moving. Do the print heads get out of alignment if the printer is jarred (set down hard or if on a cart rolled over a gap)? What's the risk of a bad label getting released (with a product) in distribution? What's the likelihood that a bad label is detected?