I agree it isn't good to generalise , but in the UK a lot of JWs are window cleaners. I would guess in my old congregation probably 30% - 40% of the working males cleaned windows and many of their wives as well. We were quite typical , I would guess. Stair cleaning is also popular , especially in the cities where there are lots of flats.
To this day when I pass a car with roof racks & a ladder , I always kind of assume it is a JW ( and often is. )
Having said that , I only know of a few who were janitors , so I guess it varies from country to country.