Trucking in the Northland

What is the round saddle tank on that overturned trailer for?  Been involved with trucks for forty years and never seen a saddle tank on a trailer unless it was there to catch refer water from a fish truck.  (most all refer fuel tanks are mounted under the trailer)

Winter is coming here too.....I'll be back to doing the chain up thing before long....

From my view that tank is under the trailer but the trailer is sitting on its side so it looks wonky.