You certainly do have a problem. You have "leakage" or a load some where. I would disconnect one of the leads from the battery bank and check current flow with an ammeter. I don't know what the amp/hour rating for your batteries might be. Let us say you have two 50 amp/hour batteries, that would give you 100 amp/hours. If you are pulling a tenth of an amp (100 ma.) you should have 1,000 hours of service, if my math is correct.
That being said, there is also an internal "discharge" that can and does happen. I have found that the older the batteries the greater the internal discharge.
I believe that, since you have new batteries, that you probably have a current draw somewhere in your fiver.