Welcome to the forum. You are one of a very few Little Joe
owners here. How do you like it so far?
Regarding the water tank. I think it is unlikely that the poly tank itself is leaking after 4 years, unless there is evidence of an incident that may have damaged it.
Likely it is a loose hose connection.
Do you get leaks
only when the tank is full, or when it is say, half full? If it leaks only on full, that isolates it to the top hoses. Check both the fill hose and the air bleed line clamps at the tank and inside the fill port. You will have to remove the fill port to check the connections there. Do you get the leaks only when the trailer is in motion, definitely top end. Note: If you pull the fill port, when you replace it,use the butyl rubber tape. I just replaced mine entirely with a locking door model.
If you get the leak when the tank is less than full, check the lower hose clamps and the compression fittings on either side of the drain valve. You should be able to wipe them dry and put something under and see if you get water. A jar lid should fit or a dry paper towel if not.