If you still have the original batteries, they are probably about worn out. AutoZone, Pep Boys, etc. will check your batteries for free. Charge them for a day or two, until you are fairly certain they are as charged as they will get. Then, have the auto parts people "load test" them. This will show how much charge they hold, and indicate if they are defective or have weak/shorted cells.
You may have one battery that has a dead cell (measures about 10.5 volts) which will allow 12 volt
devices to work marginally, but will rapidly discharge both batteries (if you have two 12-volt batteries wired in parallel). However, if both are the same age, and one is bad, you may wish to replace both of them. Otherwise, you'll be doing this again in 6-12 months when the other one fails.
Checking the batteries separately with a voltmeter may give you some indication of a weak battery or cell. However, the load test is a much better indicator of battery condition.
Note: A fully charged battery should measure between 12.0 and 13.5 volts. A slightly discharged battery will measure down to 11-11.5 volts. 10.5 and below is probably almost dead or has a defective cell (internal high resistance short, often due to sulfation).