Got 12v across the 2 6v batteries, hooked up in series. When ign in crank position 6v batteries stay at 12 volts, but he rest of the engine electrical system goes below 5 volt. Checked ground terminals with Ohm meter and loaded ground with a halogen bulb. Both disconnect switches click when turned on and off. Aux start switch does not make a difference.
Ok, a couple of questions.
1. Where are you measuring the voltage? At the battery post? If you place the multi-meter probes on the actual battery positive and negative post,(on the lead part, not on any bolt or wire or connector) and get someone to attempt to crank the engine, what do you get?
2. Did the engine crank before you replaced the battery? You mention that you purchased this coach over a year ago. Has it been started during this time?
Based on your description, and assuming you are checking the voltage at the battery post, if it was me, I would be looking at the starter. If a new battery ( presumably a good battery) has a voltage drop from 12 to 5 volts something is drawing a lot
of power. A stuck starter will do just that. Do me a favor, try starting something else from that 12 volt battery. Try jumping a car/pickup engine from that battery and see what happens. If the car/pickup engine starts fine, but your coach does not, I would really suspect the starter.
Keep us informed as to your answers, OK? Remember, you problem could be ours someday.