Thanks everyone for all of this most useful advice.
The cable to the charging circuit to the van has a VSR and a circuit breaker on it and is connected to an under bonnet auxiliary battery for in car use.
We are on the road and currently holed up at Barnhill Station, an hour or so south of Broome. This means that my procurement options are limited.
We use about 250AH per day and, with the sun shining, power is no problem.
But, even if we pull our heads in on the power use, three or four days of patchy sun sees us out of power, particularly at this time of year when the sun is quite low.
So I have ordered two 30 amp Victron units to run in parallel which are coming up from Fremantle. These will work hand in glove with the Victron MPPT controller and I hope will top the charge off when underway in patchy cloud. Total cost is about $700, which makes it an affordable option, but leaves me with a bit of a wiring job. The cable and Anderson plug from the tow are easily good for 100 amps.
When charging at idle, I think it might be best to run the engine a little faster than normal to avoid overheating the alternator. A stick on the loud pedal seems to do the trick.
I should have it installed by late next week and will report back after we get a bit of running time with it.
Thanks again,
Keith