This evening I've been out in the van playing around with the battery monitor as it stands. I still don't have my shunt yet, so can't connect it up to read current. However it should still be able to read voltage (via the INA219).
Initial results are a bit disappointing. The readings I was getting from the INA219 were wildly different from the readings I was getting from my digital voltmeter. Now, my digital voltmeter is only a cheap one from Maplins, so it could be inaccurate too, but its readings do correspond to what my Stecca PR3030 solar regulator int he van say. So on that basis I think the battery monitor is the one reading wrong :(
It would seem that the way I had connected it up to test it (via a cigarette lighter socket in the van, supplying a nominal 12v to the Arduino) didn't work to well. The reason being the ground connection. It would appear that there was a 0.3v potential difference between the ground on the Arduino and the ground of the battery terminals. This means that the ground path from the lighter socket back to the battery (via a convoluted route, including the Stecca PR3030) is not all that good. When I connected a wire directly the readings seemed a bit better, but were still quite a way off. I'm hoping that when I connect it up properly via the shunt and with a direct connection to the battery negative terminals that I will get a more accurate reading.
But the bigger... and worse news... its seems that my batteries are dead :( I switched on a light in the van drawing 1.4A from the battery as measured by putting my digital voltmeter between the battery terminal and the wire normally from it. The voltage drop as recorded by the DVM was from 12.80 - 12.63 volts. ie. 0.17v. Using Ohms law that means 0.17V / 1.4A = 0.121 Ohms. Or 121 milliohms. This is the internal resistance of the battery, and should normally be in the range of 5 - 10 milliohms. 121 milliohms is very high and indicates that there is a problem with the batteries. All three batteries of mine exhibited similar behaviour. [Thanks to eagle eyed Stewie for the correction that at 1am this morning I wrote amps instead of ohms for the resistance figures above]
The batteries are coming up to 3 years old, and have been used quite a bit, but I am pretty careful with them and they never get left in a discharged state, so I am a bit disappointed that they have died already. I was hoping to get at least another year or two out of them. So I'm going to do a bit more investigation, but seems like these batteries might have had it :(