At 12 ft it's going to be hard to get one to reach comfortably.  You will be able to get some in, but you will need a good operator.  We struggle for about 6 years with a Killbros 1150 corner auger.  Most carts are set up to dump into hopper trailers.  Here there are dump trailers with 90" sides, so about 12.5 ft.  The only carts we found that went high enough, were later model Brent or Demco.  For a cheaper cart see if you can find a Parker 4500.   We had one and that thing would dump into anything.  The auger was centered on the cart and folded rearward.