I spnt a bit of web time googling marine battery chargers that
might work with the Honda EU1000 or EU2000 generators.

Here are some URLs...not an exhaustive list by any means...
http://www.charlesindustries.com/main/ma_chargers.html
http://www.newmarpower.com/
http://www.mastervolt.com/chargers/index.asp
http://www.progressivedyn.com/
http://www.majorpower.com/newmar/p3bc.html (interesting)
http://www.batterystuff.com/battery/...o/dual-pro.htm

I like the specs shown by majorpower.com. They give a
figure for power factor, which sems to be missing from
other manufs.

As a trial...assume a 100 AH gel battery. A PT14 charger will
charge 28 to 140 AH batteries at 14 amps maximum. 14 amps times
1.1 (efficeincy of 90%) times PF of .95 (spec at 115VAC) equals
about 16.2 amps. Adjust to 115VAC input and you get about 2 amps,
or 230 VA. A 25 amp charger might work even better without over
stressing the generator.
I called a couple of the charger manufs, and they concur
with the numbers. The trick is to get a charger with a rated output
compatible with the available input power. A 150 amp charger ain't
going to work with a EU1000. But of course the trade-off is longer
charge times.

Hope this helps.
Norm B