I have a strap that I put across the back from the tie points on both sides and adjust it with the tanks in there and tailgate up holding them to the rear of the truck. I keep that strap set at that position and use it every time I need to transport by hooking it on both sides, then slide the tanks in holding the strap up and push them in to hold the strap where it is. Then when I close the tailgate it puts just a little more tension on them. I have to charge 4-6 tanks 1-2x a week, so it's gotten pretty routine. My biggest gripe has been during the winter - I really wanted to close the tonneau. At least this way you can close it up to the upper part of the tank if you want. I got even tired of that and built a canopy to cover.
Thinking about it, one of those cargo nets from an SUV across the back would probably work pretty well.