We do not use an auxiliary air purifier in our trailer, but we do in our home, and a few thoughts come to mind in considering a trailer application. First, I doubt you’ll find one that runs off 12V, so you’re limited to shore power or inverter use. Second, I haven’t run ours on our killawatt meter, but I suspect power draw would be roughly equivalent to the blower on the furnace, which means that if it’s running continuously, battery run down is a serious concern, so back to shore power. Or a generator. Which is obnoxious. Third, decent air filters are fairly sizable, so you’re going to have to carefully consider where something like that would fit in one of our smallish trailers.
This leads me back to installing filter media on the furnace’s intake directly. I did this mod, but did so prophylactically, as I had no issues prior, nor have we since, and so can’t meaningfully measure whether it’s really made any difference. It does avoid all of the pitfalls I discuss above, however, and with the added bonus of filtering all the furnace intake air, which an external purifier would not do unless you positioned it right in front of the heater. While the heater is running.
For all of these reasons I don’t think an external filter unit would be a practical solution, but the filter mod might be.
I’d be interested to hear feedback from anyone who did have problems with sail switch fouling and then did the filter mod.