Sorry but I think this is (generally) a daft idea. Range is only an issue for that <1% of trips that require an absurd distance. Let's look at this logically and I'll explain why I think it's a dumb idea:
You're a driver who makes long trips frequently - Then an EV probably isn't for you, so you'd end up using the trailer almost all the time. You're better off buying a hybrid.
You're a driver who rarely makes long trips and does not own a second car for the purpose - Buying an expensive trailer and having to deal with the logistics and trouble of storing it somewhere when it's not in use is simply not worth the hassle just to use it for maybe a couple of days per year.
you do a mix of both. In this case it COULD have a purpose, but I bring on to the more general criticisms with such an idea:
a. First up I'm going to say that I'm not American. I've seen your roads. They're ridiculously wide and when they're not ridiculously wide they are often ridiculously straight. They are split into many lanes and they're damn fantastic for massive cars, trucks, trailers, etc. I'm European. And that means I regularly drive down roads that are just barely wide enough for two vehicles to pass one another, are very twisty, with hairpin bends and constant blockages of cars and vans semi-parked up on the footpath. The idea of dragging a trailer around is laughable. This isn't even a cherry-picked scenario, you Americans have no idea how lucky you are to have such wide roads, but trailers are way less practical in most places. I probably see one trailer on every thousand or so cars I come across on the road.
b. Roads are already crazily congested, having trailers just adds to that congestion. It also makes it more hazardous for motorcycles to filter. (Lane split, it's legal here)
c. It makes parking a pain, this kind of combines with point a. our car parks are really small and have lots of tight turns too. You could go on a really long trip and end up with nowhere to park when you get to the destination, because you won't fit anywhere.
d. It looks stupid. You'd look like a bellend if you were just driving around towing a bunch of batteries. It's a fantastic way to give EVs a terrible image and make people think "oh if EV range is so bad that people need to start towing around extra batteries, I don't want to get involved, it looks tragically sad" (and very nerdy)
e. Batteries, and charging infrastructure are advancing at such a pace that spending vast amounts of money on a trailer now just means 10 years later you've got a trailer that nobody wants to buy because by that point in time nobody has range anxiety anymore. It's a bad solution to a short term problem.
f. Risk. There are extra risks associated with carrying a trailer (wind on the motorway, etc) and therefore unless you have something you specifically cannot fit into your car or van to transport, it's generally not worth the extra hassle and hazard.
g. Storage - as I mentioned in point 1. a trailer takes up a lot of space, most people live in small houses, some people don't even have a driveway.