it depends on your battery and usage. if all your looking for is something to maintain your battery, then most solar panels that will provide at least 1amp/100amp-Hr of battery would suffice. provided the panel will be in sunlight. but if you at looking to recharge the battery between uses, then that is a different story. at this point, a full analyst of your usage and capacity would be appropriate.
2000 Cherokee 29BH with 6V batteries, LED lights & 400 watts of solar power, flipped axles and raised. 2007 Tundra 5.7L DC-LB with lots of mods. C-co, 8/158th AVN Maint.