No one knows the true figures for towing, because there is no formal way of collating the figures, but there are some guestimates. For example the NCC estimate there are about 550000 touring caravans in the UK, which is less than 2% of all UK road vehicles. But caravans are generally only used for a few days per year, so the number of times a car towing a caravan needs to use a fuel station is much much lower than non towing vehicles, as a percentage of all fuel purchased towing caravans is a very very very small percentage of the total fuel\energy purchased.
Of the other types of trailers used, most will be doing very short local journeys, and will probably never be taken to a public charging station.
However, caravans will be different, until EV batteries gain a lot more capacity, most caravanner's will need to use enroute charging facilities, so there is a need for the charge providers to give more consideration to being able to accept towing outfits.
This is an aspect that the caravanning club's, and trade bodies like the NCC should be lobbying both the charge site providers and government about.
Even individuals could do something like writing to you MP about the lack of space at charging sites, which might render the trailer open to theft if it has to be detached from the towing vehicle. Half a million caravanner's could make quite an impact and cause the gov to introduce certain planning considerations to future sites.