It is extremely difficult to read your mind, so please give the proper information to be able to give you suggestions.
Sulfation will occur during discharge and during discharged times and the effect depends on the type, quality and of course ABUSE.
Most People MURDER their batteries, and You are doing it "VERY" WELL.
LVD trips but for Us is tripping in the dark since you are not indicating what the trip voltage is, to know the depth of the battery discharge level which can give you an estimate of the "sulfating" problem.
A 17 AH "deep cycle" battery under heavy load will "deep" down fast which forces the LVD to trip, also the internal resistance (impedance) of the battery is a bit high because the AH is low.
Solution: Increase the storage capacity of the battery.
You should have at least a 50 AH battery, you have 50 *6 hours = 300 watts divided by 12 Volts = 25 Ah * 2= 50 AH capacity.
75 AH may be a better solution.
Nando