Originally Posted by Tford
Question for long time owners, is there much metric sized stuff in our trailers? I know there's lots of square head screws and such. But then again, most "US" cars/trucks have metric doodads here and there from manufacturing in Mexico and the French end of Canada (my "mise en garde" radiator fan)...
"US" cars are largely (now perhaps entirely) metric, and it is unrelated to the assembly location. The sales
location drives bilingual or multilingual labels, regardless of the measurement units. Nothing newly designed in the last three decades or so has been in inches, although some long-running components (such as engines originally from the 1950's still in production decades later) still had inch fasteners until they were redesigned, and there may be some left in use. This led to the annoyance of needed dual wrench sets to work on one vehicle; this was really bad in the 1990's, although I suppose that's pretty well gone now. I would be more certain if I had ever owned anything really "domestic" from this century, other than my motorhome chassis; others may have current examples to share. I had a U.S.-built 2004 Ford Focus, but that European design was always entirely metric.
On the other hand, the trailer industry is firmly entrenched in the 1960's (or earlier) in most ways, and all the trailer hardware I've seen uses inch fasteners; certainly the bolts holding the axles to the frame and the wheel studs and nuts, for instance, are inch-sized. Threaded battery terminal studs are still inch, too. Propane fittings have inch-sized hex sections and threads. The only place I can think of where metric wrenches would be needed might be in an appliance... could be anything inside those things.
The Robertson (square drive recess) bits are neither metric nor SAE (inch)... they're just a proprietary size defined by Robertson, so there is only one version.