I was actually thinking a government run program for the distribution and thinking that the scale of producing these things would be such that they would be cheaper (eventually) than the average home/small business capable generator would be.
You're looking at extra equipment, things like highway speed capable wheels and axles, fairings, more electronics, etc... No, they're not going to be cheaper than the average standbye generator, which already has world wide production levels to have good economy of scale.
To put it another way, the same Briggs&Stratton engine can be put into generators, power washers, lawn mowers, pumps, and many other devices. They get their economy of scale from there, not from the specific frame.
On owning the trailer - Not a big deal if it's doing double duty as a home generator. Keep in mind that it's not the trailer body that's expensive in this case, it's the generator itself - which runs $4-5k, given that we want something that can run all day with good efficiency.
Oh, and the $3k for the steering upgrade is it's cost some time ago - if installed on a lot of trailers it'll be substantially cheaper.
An attached generator would also be lighter than one that is towed which would help with efficiency though a trailer does not decrease efficiency _much_ when it is in motion though it is horrific in stop-and-go traffic.
Not necessarily. Remember that you need to reinforce stuff to withstand being attached at a single point, not to mention the drop down legs and stuff for when you remove it. As for stop&go traffic - remember that that's where EVs shine with regenerative braking. Trailers on traditional vehicles suck in stop&go because you're scrubbing all your kinetic energy every time you stop, and that's proportional to the weight of your vehicle and any trailer. With an EV at least a portion of that ends up in the battery instead.
Though I agree - the common case would be to drop the trailer as soon as practical once you get into town, before doing 'lots' of city driving.
Theft could be an issue, but as you say, there are ways to secure things.