Costco is a great place to get the batteries.
But before giving up on those, try looking them over and filling them with water, and see if you can charge them. Chances are that you will never get more than about 60% of it's new performance, and you might want to skip that step if you have already decided to buy better batteries.
Golf cart batteries can stand a lot of abuse, but leaving them at less than 10 volts is a problem that no battery can recover from at times. 12 volt batteries are not nearly as likely to withstand any abuse.
A 100 watt solar panel will keep them full while in storage, if you remember to turn off the propane detector and CO meter. Those two alone will use about all the power a 75 watt panel will produce in a day.
SunElec.com sells a 140 watt panel for only about $229. You can mount it yourself, and wire it with some from Home Depot (Grey direct burial wire #10 rated at about 15 amps limit and can be exposed to the sun).
I made mounts from 2" angle aluminum that is 6" long and 3 holes for #10 screws into the roof, and 5/16" hole for a 1/4-20 bolt into the solar panel frame. It holds the panel about 1" above the roof, so cool air can circulate under the panel, it will put out more amps when cooler.
Good luck,
Fred.
