FWIW, back in the sixties when I worked part-time at Lockheed while a student, an engineer there showed me a battery the size of two packs of cigarettes. He said it would turn a car engine over for 10 minutes and cost around $10,000 (in back-then money) each. It was based on some exotic chemistry I never heard of and was used in our cost-is-no-object space projects at the time.
Actually, the most perfect battery - that will last "forever", can be taken to zero or any other voltage with no damage, and do this an infinite number of times - is a capacitor. You just have to figure out how to pull one large enough behind the RV in a trailer so as to equal the amp-hours in a common liquid LA RV battery or in a common up-coming lithium RV battery!
