You guys all seem to drive right over the cliff, right off the bat.
Start with disconnecting the battery/batteries in question. Check the voltage of the batteries in question. If they're less than 12v, put a separate charger on them until they read proper voltage. If they don't ever get to 13 or so volts, change them. If they do, test them with a load tester. If that test shows bad, replace. If it's good, reconnect the battery, and see if it will start. If it does, you're done. If it doesn't, you have a wiring issue that needs to be addressed at this time.
Work smarter, not harder, and start from the known and go to the unknown. That is when you will find that problem, and have a better chance of repairing, finding, and fixing it.