Running your van engine to charge the batteries is rather wasteful, both of gasoline and of engine life on the van. I've a little ECQ 1800 watt inverter generator that will run for about 5 hours on the quart of fuel in its tank. I've also got a 5500 watt open frame generator that will run for at least 12 hours on 7 gallons of fuel, a lot longer (but I don't know how long) on a light load such as a 30 amp battery charger (which, after all, claims to provide only 360 watts worth of charge to the battery). What you did is in some ways equivalent to hitching up a full-size bulldozer to pull up your tent stakes.
I'm thinking that the reason that your battery charger took so long is the MSW inverter you used to feed it. Obviously I've no knowledge of the circuitry in that charger, but I suspect that it would be much faster if feed from a pure sine wave power supply. Most battery chargers of the automotive sort use a transformer to reduce the voltage to 12 from 120, (at least the ones I've seen), and a transformer is only going to provide power during the rise/fall time of the waveform, meaning that for most of the cycle, they will yield nothing, thus being very inefficient.
I quite understand the costs involved in buying a suitable small generator. I passed through a Northern Tools store just yesterday and saw that they had a pile of 2000 watt inverter generators of their own Powerhorse brand claiming to run at only 52 dB noise level (which is quieter than Hondas, albeit not by much) for only $599.
Do be aware that for running that battery charger, even one of those tiny 900 watt, 2 cycle, $99 Harbor Freight generators is quite adequate.
