Another 12V charging question - with a twist!

I have already spent a small fortune on my AGM batteries (2 x 120Ahr) and would like to make sure they are fully charged while I am travelling. However, the cost of a 25A dc-dc charger can be up to ~$500. Now, I already have a 25A - 240V multi-stage charger for landpower charging and a 400W pure sine wave inverter, so its got me thinking...

... why not connect the 240V charger to the inverter and run the invertor off the vehicle alternator while travelling? The 240V 25A multi-stage charger puts out 25a x 14v = 350W at full power. Lets allow 20% conversion loss and you have a max draw of 420W, less if not putting out the full 25A. Now, my inverter will put out 400W continuous and will allow an overload, so even at 420W it should still run.

OK, my current equipment is on the borderline of running this, but if I upgraded to a 600W invertor I would be able to fully charge my AGM batteries from the alternator. The only thing I would have to do is unplug the 240V charger form the landline outlet and plug it into the invertor power when travelling. No hard wiring changes at all. In fact, you could probably even get a switchbox that selected which 240V power source to use if you wanted to get fancy.

OK, this is not the greenest way to power things, there is the loss from 12v to 240V and then from 240V back to 12V. But even a dc-dc charger has some inefficiencies and given you are saving the cost of a dc-dc charger, the inefficiency just means your alternator needs to work a tad harder. It also means you only need 2 items (charger, invertor) and not 3 (charger, invertor, dc-dc charger). Overall, I reckon this is more green - definetly cheaper if you already have the stuff :)

Now, before I patent this method, is there any flaw in my logic of doing this? I do realise that the invertor needs to be wired to the vehicle alternator and not to the van batteries (now that would be real green power!!!).

Cheers

Captain