Two Group 29's (right around 200 amp hours and you can safely use about 40 of 'em) should work with a 1500 watt inverter 3 would be better but it's more a factor of load and you are under 1000 watts load (closer to 500) so should not be a problem (200 amp hours per 1000 watts of inverter is Xantrex's suggestion) I think you will be OK.
Now if you already have the batteries.. OK
When you get around to replacing them consider GC-2 golf car batteries These are six volt so you hook 'em in series to make one BIG (4d big) 12 volt battery
Around 220 amp horus and you can safely use half (Note the difference you went from 40 usable to 110 nearly 3 times the usable power)
They may need to be "Shimmed" a bit to fit in a group 27 box and they are Taller so measure before you upgrade.
They also need to be watered from time to time (DISTILLED ONLY)
They may also cost less than a pair of 29's
But as I said. if you already have the 29's they should be good till they fail (NOTE folks. all batteries fail sooner or later)
Make sure you have a good 3-stage converter to charge 'em with. I like the Progressive Dynamic Charge Wizard controlled units... Have yet to find one better.