Harbor Freight offers a 1.5 watt solar charger for $20. It plugs into a
cigarette lighter socket. They are also offering a 5 watt unit for $50.
This
one clips to the battery.. They also have an 18-24 volt model for $140 and
a
16 watt unit for $180.
5 watts probably won't do the job of re-charging. It might be able to keep
the 2 batteries topped up once they were fully charged by some other means.

A 5 watt panel would probably output something like .4 amp under perfect
conditions. Depending on where you are, perfect condition are probably an
hour a day, with less than perfect for 6-10 hrs a day and no charging the
rest of the time.

If you assume that you get 1 hr of optimum charging = .4 amp/hr
plus 8 hrs of less than optimal @ .25 amps = 2 amp/hrs
Total 2.4 amp/hrs per 24 hour period.

At this rate, assuming your batteries are 50% discharged, it would take
22-23 days to re-charge them if batteries were 100% efficient when charging,
but they're not, so figure 26-30 days to recharge both batteries from 50%
discharged.

In summary - you will probably need more panel output if you want the panel
to recharge rather than maintain a charge. I'd suggest 20-30 watts might be
more like it as a minimum.

Optimum for solar panels is when the sun is directly shining on them at full
intensity with no shadows at all. When the sun is lower in the sky the
efficiency is lower, any shadows may reduce the output drastically.