I in fact have a Harbor Freight 12 volt load tester and it has worked for years testing hundreds of times. Sometimes they are on sale or have 20% off coupons in their circulars. I don't have a clue about the bad reviews. Maybe they had a bad batch. Mine has worked flawlessly. It is a resistive coil in a metal box with a built in gauge and a toggle switch to apply the load through the resistive wire. Similar to turning on a toaster which runs current through wire coils to heat your toast.
The tools that I previously mentioned are still good to have on hand in any case. AutoZone or O'Reilly can tell you if your battery is good and is performing as to it's rating. Others that I know, say that the warranty is not as good on the Dealers battery as a replacement from AutoZone. You more than likely have a 3ish year old battery. Did you put it on the charger to top it off to see it's readings? Attach your 12 volt, charger with black lead to good metal ground (not battery) red to battery positive, check your settings, then plug it in to the wall. When done, unplug from the wall first then remove charge leads.