If your rear bag is too small and requires too much tension to get steady then you may need one that is both larger and stiffer. One of my favorites for a long time was a simple old pair of boot socks with a bag of rice in them. I now use a Reasor Precision Game changer bag pretty much universally regardless of where it is supporting the rifle. Shooting out of a truck bed is not the problem. Getting steady (even in the prone) can sometimes be tough. Correct bipod height and rear bag combo play into this equation. Were you missing consistently? high/low or left right? If you were all over then it is most likely due to the shooting position you built. Figure out to get rid of the wobble and you will find the target gets easier to hit/spot misses, and adjust accordingly.