They don't exceed highway speed so swapping batteries every X miles will be worth it if they reduce total costs by doing so.

As to weight the engine weighs ~3000 lb and 300 gallons of gas weigh ~1,900 gallons. Transmission is another 700lb, include tank weight and verious odds and ends and you can get ~5 hours at highway speed. Assuming a battery swap takes 10 minutes. That's easily worth saving 50+k per year in operating expenses.