Unit sales or some other metric such as market share would have to be used because FR is a subsidiary (owner is Berkshire Hathaway) so it does not have a publicly traded stock.
This article says Thor is the largest manufacturer, but I don't know what they are using to make that claim.
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Scott
DW, 3 Kids and our Goldens
2012 Shamrock 233S
2008 Toyota Sequoia 5.7L 4WD