The "reserve capacity" usually refers to an SLI (car starting) battery not a deep cycle. A quick look at that shows a 60AH capacity for the battery, perhaps yours is higher if it is a deep cycle. But with any AGM
battery, if you pull it below 50% capacity you shorten the life. Below 70-80% capacity, you shorten it even more dramatically.
And if you have an SLI (car starting) battery, deep cycling it radically shortens the life, no matter what kind it is.
Since "watts is watts" and ignoring the ac/dc conversion factors, let's say your ice box pulls 4 amps, times a six hour cruise
that's 24 amphours. No problem for a deep cycle battery with a 60AH capacity, no problem for an SLI battery as long as you don't make a habit of it. SLI batteries are mainly designed to never be used for more than 10% of their capacity, as few as 5-6 deep discharges (80-90%) can ruin them. Those kind of numbers will vary with the maker, the battery type, and how long the salesman's nose grows as he tells you about it.
If you really want to make sure you have no power problems, see if a supermarket or bait shop near you sells dry ice. Stick a bag or two in the cooler before you go, and it may not even turn on during the whole trip.