I say just go try spending the weekend. Your start batteries are likely isolated from the house batteries, so go enjoy a weekend and see how it goes, knowing you can always start the engines. My house set up is a mere 2 golf cart batteries and I stay out for 2 to 4 nights without a problem. You will be leaving the dock with your batteries fully charged up from shore power, so the only time your alternator will matter to you (based on your question) is when you are out for multi nights (2 or more) and moving between anchorages.
You can save a ton of guessing and over thinking by grabbing the small Honda gennie and taking it along and forgetting alternators altogether. I have one and almost never use it. I also have a two year old who likes to open the fridge and look inside on hot days.