After what I personally had to experience on 9/11, I've made a small donation to my city's PD and FD every year in September. If the local (say) Taco Bell wanted to show financial support for the local FD, there are no regulations that would prevent it, yes? I suspect they could even put up lettering on their sign saying "We support (this town's) FD" simply by stopping by the stationhouse, writing a check, and asking if the sign was OK.
Some FDs have fundraisers, others don't. Politics even at the municipal level can get icky...I've heard that some PDs and FDs aren't fond of fundraisers out of concern that fundraisers could reduce their negotiating power with City Hall. I don't know how true this is? If this is true, then I can't see how selling ad space on the trucks would be an improvement.
If nothing stops a business from making a routine donation to an FD, the sale of ad space means the business wants more than just community goodwill....they specifically want their ad to be seen. (I think the PETA example makes this point clear).
Fire trucks are not out in the community for routine matters, the way a police cruiser would be. If the ad space (the truck) is seen by the target audience, that means ad space is most likely involved in an emergency.
This means the municipal government is now profiting from its citizenry's misfortune.
Is this wise?