I assume that Leisure Battery is a Sealed Lead Acid battery (SLA). If you discharge a 12V SLA to lower than ~11.5V, then you have permanently damaged it. Read the maker's data sheet. During charging, the voltage should not exceed 14.4V.
Your voltmeter should indicate the range from 11V to 15V, and you should stop discharging the battery when the voltage drops to ~11.5V.
I hope you are using an efficient Buck Switching Voltage regulator to get from 12V to 5V. If not you are wasting more than half the available battery power.
Since your final load is 5V, I would have started with a 6V SLA. That would have made the voltage conversion process even more efficient.