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Start by having the batteries load tested and clean the cables good including the ground lead to the chassis. This sounds like a battery/cable issue. If that all checks out we can dig deeper into this. You should have a small digital multimeter for some testing if we need to do that. About $20 or so at most hardware stores and easy to use. Get back to me if we need to do that. If you remove the batteries for testing be sure and get a photo of the hookups first so you get them back in the same way. Easy to get this wrong if you don't. Most places that sell batteries can load test them for you.
Just checking back to see if you were able to get this resolved or do you still need some additional input from me here?