Okay, Ill put my numbers in, and show my work.

It's interesting, and perhaps alarming,that no one has arrived at the same answer yet, although some are close enough to the same for practical purposes.

  Numbers in red are just numbers.  All other values are dimensions in ft.  Green values represent height, black represent x, y dimensions 

Struck:

((((12*18)+(8*16))/2)*2)+(8*16*1)+((((8*16)+(2.5*15))/2)*3) = 720ft3



Heap:

12*18*(6*tan(25))/3 =203 ft3





Total 923 ft3



Just realized I figured the heaped portion incorrectly as a pyramid...but
the difference is going to be marginal so I'm going to just defer to
those who have figure it correctly and adjust upwards 32 ft3 to get 955ft3 total.



I am thinking about 41 lbs /ft3, based on figuring volume and known scale weights on our existing bed,, so that is  38250 39155lbs