I would just figure on installing new batteries in the Spring. At least you will get the core charge from the old batteries.

To load test them measure the voltage after they have been off the charger overnight. Then connect a 50 watt power resister across the terminals. That should draw around 4 amps. Leave it on the batteries for 4-5 hours (note how long). Remove it. Let the batteries rest overnight then check them again. You can get a first approximation of their capacity from the extent of discharge.

i.e., 4 amps x hours connected = amp hours consumed

(amp-hours consumed)/(1-(final voltage - 10.8)/(initial voltage - 10.8)) = capacity in amp-hours

For example lets assume the batteries start out at 12.8 volts and the resistor is on the batteries for 5 hours. Amp-hours consumed is then 4 amps x 5 hours = 20 amp-hours. If the batteries final voltage is 12.2. then the approximate capacity is:

20/(1-(12.2-10.8)/(12.8-10.8)) = 67 amp-hours

This assumes that the batteries are fully charged when you start.

Note that the power resistor will get very hot, so make sure it can't touch anything flammable. 12 gauge wire will be adequate for the connections with a 50 watt power resistor.