1 you don't have deep cycle batteries you have poor marine batteries. The 20 amp hour rate is what you should be paying attn to. On a 6 volt deep cycle that should be 220 amps plus
You have one battery? Or 2 or 4 ? That's not clear to me . Read online "the 12 volt side of life " to give you a better understanding of how all this fits together.
If you have 400 amp hours ( 20 hr rating 4 batteries then you can use a maximum of half that safely so you have 200 useable .Now if your using 10 amps of Drain that's 20 hours of use if your using 20 amps then only 10 hours