1kwords said:
So, if it says Deep Cycle (like the one I just bought), it may not be a true deep cycle? It's so confusing ... what's the difference?
The difference is the constructions inside. An automotive battery has many thin plates to give Cranking Amperage to the system. More thin plates give more surface area which allows a large supply of energy all at once.
A true deep cycle battery has fewer but thicker plates inside. Remember I talked about more lead and heavier weight before? These batteries allow smaller loads but for a longer time. Nothing in your rig uses a large amount of electricity at one time like the starter on your car/truck, so this design is perfect for smaller, longer draws of energy.
As a battery charges and discharges small particles of the lead plates flake off. The thick plates of the true deep cycle are designed to allow this to happen for a large number of cycles. The thinner plates of an automotive battery are not designed to be deeply discharged, and will fail if deeply discharged many times. The further down towards dead the battery is discharged the more of these particles flake off. This is why it is recommended to not draw a battery more than halfway dead.
A "marine" or "marine/RV" battery is a battery with a construction in between the automotive and the deep cycle. It will not allow as many discharge/charge cycles as a true deep cycle and it won't give as big a burst of energy in starting situations.
However, having said all that very few people take good enough care of their batteries to get the optimum life possible anyway. Many people draw their batteries below a half discharge, they don't water them properly, and they don't pay attention to charging the battery and keeping it topped off properly.
Your specific problem, size of space available for the battery and the battery types readily available that fit in that space may make the marine/RV battery a reasonable choice for you. The AH rating is probably as much as you could expect. Paying attention to the maintenance and charging of your battery should give you a reasonable service life. Each of us uses our equipment just a little differently, and we all have to find what works best for us. If this battery provides power for the 3 days you need and you get a reasonable service life, then you've found what you need.
Ken