[Nyororin]

I have never encountered any trucks with THAT direct of a slogan.
They are fairly common on national holidays (I say common as in a few trucks will be out driving around) and on certain occasions some of them will gather...
But their slogans are usually not quite so direct. I don`t know exactly what the truck your friend encountered was saying - and it probably could in a round-about way be interpreted in that way... But in general the slogans are more patriotic than outwardly attacking. Of course, usually patriotism overdone to the point of being racist, but not quite as direct.

I have NEVER encountered anything worse than a few trucks playing loud slogans or patriotic songs from war time. No gatherings or actual hostilities. Most people just ignore the trucks and consider them incredibly annoying and obnoxious. It`s also pretty common that the people driving them have nothing to do with the groups putting them out there and are just hired to drive them around.
I can`t possibly imagine there ever being a "rally" of the sort.