i disagree
I disagree with the signguys that are against this idea. That mindset is the same one that was against vinyl originally, and then against FastSigns, and they are now the largest sign firm in the world and vinyl is now the goto for most profitable shops. Do what betters the customers life and you'll succeed.
The business model youre asking about is the same a tool truck. Ive studied this model several times over 20 years and am building one out now. My banker loves it for SBA. Tool trucks charge very high prices for the CONVENIENCE of going to the customer. They understand what their customers want. That convenience is cost effective for smart buyers. To letter one vehicle requires it be delivered to the sign shop and another vehicle & driver be available to shuttle the first driver back. For a tow firm, that can be as high as $400 if they miss just ONE rotation call. Now repeat this time loss to pick it up. Very quickly the time lost going to the sign shop is more expensive than the job.
So for vehicle lettering there's a viable market IF the service is PREMIUM PRICED and you sell the customer's lost time as the savings. PREMIUM SERVICE=PREMIUM RATES. *** Remember FedEx isnt $0.47 per letter like the USPS.
Premium service requires you NEVER dilute your offering with the junky look many mobile sign trucks have. You need to brand the operation as the local premium provider, and only use premium films, and deliver premium design. (Dont bother with this idea if you're inexperienced, lack a graphics arts degree or journey level career training. the result will just be a failure). Like any business, if you look like junk, you'll be perceived as junk and never will get out of that rut.
As for signage, premium fees can apply for ANYTHING that requires the brick n mortar sign shop go out to compete. Most shops charge for a local trip and for a minimum order. Know those rates an ADD them together - The Mobile model works if you have a combined minimum thats equal - but only for services that the sign shop has to send a truck on or where the client has to be INCONVENIENCED to get to the sign shop. So window lettering, truck lettering, car striping & graphics, maybe even color change wraps. NOT banners, rigid signs, custom signs, etc.
Mobile is a SERVICE, not a product, and if you are top shelf, many larger sign shops will likely be willing to refer you.
Whatever you do, keep a solid inventory. One mobile service i am aware of had to make several trips over several days to letter a clients tow truck because he didn't have orange on board. WTH? orange.. The customers impression of the guy was that he was a flake, and when I heard what he charged (about half the going rate for the market) it was obvious he's clueless.
Study the franchise offerings from Matco, Snap-On, etc. see how they work and follow it. So long as you stick to a tool truck model you would likely do fine. If you try to compete with bricks n mortar shops, you'll get burnt.