Lonne,
Since you generator exhaust runs out the back, there are no Beaver dry camping rallies where a Genturi would be necessary since we do not park coaches back to back at our dry camping rallies with few exceptions, and there are many parking options at our dry camping rallies like Quartzsite. The coaches that benefit from a Genturi are the ones with a side generator exhaust outlet. However, if you go to some other dry camping rallies, especially at a crowded area like a race track, you may have a need.
With you battery plans, if you are not having a problem like running through the night with a residential refrigerator or a C-PAP, you will have very little benefit from increasing you battery bank. You will not decrease your generator run time, you will just have to run it longer but less often since you still have to replace the same amount of KWHs, and you inverter will only recharge the batteries at a preset rate.
Gerald