solar charging question - adding additional battery

Its a 55amp hour. Optimal blue top AGM. It would be hooked up to the trailer batteries with like jumper cables. It would be about 4 or 5 times a summer when im on long trips where i need to recharge the boat battery in order to fish more! It lasts for 2 or 3 day trips but then starts to die and needs recharging. I have a 60watt portable solar kit i could hook up to it, but using the 400watts from my trailer would be so much faster to recharge.