What is the capacity of the batteries n Amp Hours (Ah) that you intend to purchase? What is the average power consumption in Amps (A) of your fridge/freezer? What is the rated output of your alternator in Amps (A)? To illustrate the calculations let me make some assumptions about your system, if the batteries have a combined capacity of 270Ah they can be safely discharged to 50% so you will have usable capacity of 135Ah before having to recharge. If your fridge/freezer consumes an average of 3.5A it will consume approximately 84A in a 24 hour period (perhaps better in winter ambient temperatures). An alternator rated for 75A may produce 60A in bulk rate after heat losses and inefficiencies. If the engine was ran for 8-9 hours in a 24 hour period it might be able to cover the anticipated usage of 84-90 Ah a day. If your specs are bellow my assumptions you will likely fall short. Now, if you would spend the night at a marina every 3rd night and have a chance to recharge batteries 10-12 hours from shorepower you should make it fine. Understand that the figures reflect a safe discharge limit of 50% for the batteries as your criteria is not to unduly harm them. There are things that you can do to reduce the power consumption of the fridge/freezer like packing pre-frozen items, limiting the number of times the lids are opened and pre-cooling the unit on shorepower prior to departure. A full fridge is more efficient than a half full one. There are other tricks like freezing water jugs to use up space as well as helping keep temperatures. Find the specs of your system, run your numbers and determine your approximate consumption and production and if and when it would be appropriate to spend a night at a marina. Hot showers and Restaurants are an added incentive.