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My bet is the guy who put the 8V battery in it first put 12V battery cables in it. These are smaller diameter cables and are not sufficient for 6V use. So he then decided to go 8V. If he had used the proper cables to begin with the truck would have been just fine on 6V. Just my guess.
If the 8V battery is needing a constant charge it's probably due a replacement anyway. My advice would be replace it with a 6V, replace all the cables with 6V cables and be sure all your grounds are clean.
