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I have heard of two propane tanks being on a F-series, but if the truck was modified to propane only and you removed the gas tanks can you add two more propane tanks in their place?
If I remember correctly you cannot exceed 300 gals on a commerical vechicle and 200 on a consumer. If you got the room mount them but you must hook them up so they do not cross fill. That is when a bigger tank over fills a smaller one.