You said it was slow to turn over. That would indicate poor batteries, a binding starter motor or a seized engine. Well I think we can discount some of that. Glow pugs are pretty unnecessary here in the UK it doesn't get cold enough and wouldn't make it turn over slowly.
Eliminate the obvious. And that is batteries. If you put a voltmeter across even a dying battery you might still get 12v or so out of it. It's load that is important here. There is no substitute for one of those great big resistor battery testers. Get a garage to stick that across the terminals and see if it glows. Old fashioned but fool proof. When the batteries are connected together like this, there is a degree of levelling so yes they will both read the same unless you disconnect one end and test them separately. All of the time they are sitting there not running, the good battery will be feeling the effects of the poor one. It can't 'charge' it as such, but there will be some movement. Then when you turn the key, the poor battery is like a sort of resistor (I said like, not is guys) placing extra strain on the good battery.
The quicker you find it the less likely you will be to need tow. But you may be two late. They go down hill rapidly. Think of the good battery supporting the weak one and 'covering' for it. When the weak one goes, the good one will be exhausted from propping it up and in all likelihood will fall over too. If the batteries both pass the test then it's onto the next thing. I think that there must be a mile long queue of Colorado owners who have been through this though so as I said, worth checking first and it doesn't usually cost. There are electronic battery testers but frankly I'd rather use the old prongs and heater element device.
