What have you estimated?
Have you ever tested how much water "YOU" use per day? Have you ever gone on an extended cruise and measured exactly how much water you use? For drinking, washing, brushing teeth, cooking, washing dishes? If you haven't done that, you should. Everyone's water usage is different, so other people's estimate is of no use to you. To measure it is not difficult. Just use a gallon jug. Fill it with the water from your water tank, and only use the water from the gallon jug for everything. Every time you have to refill the gallon jug, log it. And you should do that for a few days. If you do it for only 1 day, then you will subconsciously defer things until the test period is over. Like taking a shower or washing clothes.There are also a lot of things you can substitute use salt water for, like washing dishes, then just a quick rinse with fresh water. You can use salt water for cooking too.The same answer applies for food, batteries.There is no such thing as “on average”. Only average for “YOU”.As to living on a 41’ all year around? Of course you can. So can someone living on a 25’. There is a family at our marina doing just that, with 3 kids on a 25’. I sure wouldn’t want to do that. So what are you comfortable with? 25’, 35’, 45’, 55’?No one can really answer these questions for you. Sorry.