Engine will only start with AUX Battery Start

It seems that a common approach to troubleshooting 12V (and other systems) is to replace the most likely failed part (in this case the battery) without applying some logic. This is often an expensive and frustrating approach. I've learned the hard way to ALWAYS check the stuff first that is free to fix. In this case, clean and check all connections, and using the voltmeter described above, rule out bad cables. If you suspect the battery, don't just blindly replace it, make sure it's full of electrolyte and fully charged, and get it load tested. (that's usually free) Also, check solenoids, which can by bypassed with a jumper cable (be very careful, they can carry a lot of current). Also, check fuses. My C had a 5 amp inline fuse to the charging solenoid that would occasionally blow and the solenoid wouldn't work. My point to this rambling is that with almost any RV system there is a lot of basic, FREE troubleshooting you can do to pinpoint a problem without replacing any parts.