Once you get up into that sort of capacity, the chargers themselves will run you 5-600 bucks. While you are at it, you might consider going with a charger /inverter like the Xantrex Freedom line which is only a couple of hundred more. This would save you the trouble of installing a separate inverter down the road and simplify the installation. With 750 a/h's you can go as high is 150 amps on the charging end with a minimum of about 75 amps as Dawg says.
Your 50amp will be fine if you just do weekending and remain plugged in most of the time but the above comments apply if you are out cruising or fitting out for it in the future. You have about 375 a/h's of WORKING capacity before you need to recharge your new bank so it is really a question of how long you have to do that.