Find out what the sparky was quoting you for. I've been looking into this over the past few days. Here are some rough costs.
Battery - $300
Quality DC-DC charger and isolator $200 - $300
Quality cabling, lugs, fuses, breakers - say $150.00
Labour - well to get $1000 that will be a charge of $250.00
So it may not be too bad a quote.
Yep, what he said is correct, modern alternators, post 2011 (and this is what I've picked up over the last few days) output a higher voltage on startup and detect when the starting battery is charged. When it is they drop their output voltage. It's this dropping of the voltage that stops the aux battery receiving enough time to charge.
I've just spent some time today looking at DC-DC chargers. I've been looking at 20A chargers, as many brands as I could find. They seem to range from the cheapest I found was an Aussie made one about $160 to Redarc around $350.00.
Cheaper ones are heavier, bigger and may require manual setting up. Expensive ones are smaller, lighter and easier to setup.
If you're starting out and plan on doing any type of camping, I'd get this done correctly first time up, I don't mean get the sparky to do it if you can, but get the right bits and pieces.
$600 for the Redarc isolator is probably the BCDC1240 or larger. That could be overkill.