Did you use one of those BEP switches to control the battery isolation? Not sure what the Car Toys guy is talking about if you've got your radio still hooked to the starting battery.
It doesn't matter what type of battery you used, capacity is capacity. The difference is in load type tolerance. Marine batteries don't mind long duration, small loads while starting batteries hate them. OTOH, true deep cycle batteries won't last long starting engines. That is due to the differences in construction methods. There are some difference in how deep you can discharge repeatably the various types. But regardless 100 A-hr is 100 A-hr.
If you want a true wired back-up, a battery switch is the easiest solution to that. For that matter, you can mount the battery, use a passive isolator to piggy back to the charging system, essentially trickle charging the back-up when you're driving, and carry a pair of jumper cables to save yourself. The reverse resistance of a diode is very high so it's for all intents and purposes separate from the truck, it's only consumes much current and supplies next to zero.
It's when guys want automatic and smart controllers where the complexity increases. If you have a manual switch that disconnects the second battery and you have nothing hooked to it the only mechanism is self discharge, which is true of all lead-acid batteries. Left long enough the internal resistance of the batteries itself is a very small load that discharges the cells.
'91 Toyota Pickup
'09 Kawasaki KLR650
'12 Gunnar Rockhound 29
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