What type of batteries do you recommend?

Most of our customers prefer to use deep cycle marine batteries with their inverters. A few advantages are: Delivers higher peak amps faster than conventional batteries
Provides up to twice the life of conventional batteries
More consistent voltage across the discharge curve
Superior cold and hot weather performance vs. conventional batteries

We suggest Lifeline deep cycle batteries, they work extremely well with power inverters.

 The following three paragraphs are from OPTIMA Batteries
Connecting Batteries Together in Parallel
If your battery application requires more starting power or reserve capacity you can install multiple batteries together in parallel by connecting the like terminals together (positive to positive / negative to negative). Each time you add a battery in parallel you increase the CCA and Reserve Capacity, the voltage remains at 12 volts. For example, two OPTIMA? 34/78's in parallel will provide 1600 CCA and 208 minutes reserve. Three in parallel provides 2400 CCA and 312 minutes of reserve. If you have any questions about multiple battery installations, contact an appropriate automotive service center.

Suggestions for connecting batteries in parallel
Use batteries of identical make, model, and age.
Make sure cable gauge is sufficient to handle the higher current flow.
Prevent cables from shorting (do not allow them to rub against the vehicle body).
Use only high quality connectors, clean all contacts prior to installation.
Periodically check all connections for snugness.
If you are unsure of this procedure, contact your automotive service center.

Battery installation tips
Ensure that the battery is properly secured in the vehicle or equipment to prevent movement or vibration wear.
Do not over tighten the hold down bracket
Connect high amperage accessories such as a winch to the top terminals only. Do NOT use the side terminals.
Replace any cables and connectors that have corrosion, rust, or other damage. 
Do not install batteries in a non-ventilated or sealed compartment.
Do not lift or handle the batteries by the terminals.
Do not over tighten terminal bolts