When I moved a 100 gallon, I used a thick blanket on the underside to keep it from rubbing against my plastic bed liner. That actually made it easier for it to slide around so I had to use whatever I had to cushion the sides. I had a couple of plastic tubs that I put around the sides and I used some ratchet straps to make a net of sorts to keep it from sliding back and forth. The ratchet straps had just enough tension on them to keep them in place but not enough to put unneeded stress on the tank. That worked just fine but I still had to drive slow and stay off the highway.
Next time I move a big one, I'm going to use a rubber mat on the bottom and might even take some wood and tools to frame it in somehow.
The biggest thing I would be worried about is it sliding when you accelerate or brake as well as it maybe trying to fall over on curves or tight turns.