I would fully charge the batteries, then have them tested with a load tester. Most farm and car dealers have a load tester. So do most auto parts stores. I'd have the alternator tested also. If they check out okay, And the batteries won't stay up, you can be pretty sure you have a power draw somewhere. Eliminate all the simple stuff first. Like a couple of bare wires inside the wiring harness, bare wire rubbing metal, that kind of stuff. Good luck and I hope you are a patient man.