You check the sg with a hydrometer. An inexpensive one will work for you. It should be about 1.265 if fully charged. Wear eye protection, you are working with acid. ALWAYS have water with you to flush your eyes if needed.
HydrometerYou also need to check the battery voltage while it's charging with a digital voltmeter. Are you charging with the house converter and what manufacturer or model is it?
If it only goes to 13.6 volts, it will take forever to fully charge.
Read what "the 12 volt side of life" says about battery charging in RV's.
12 volt side of life