Look up the owners manual if there is one available. There may be a switch to turn on or a breaker. Turn on a light switch and put a voltmeter across the battery to see if the voltage is holding up. It could be a very bad battery.
While on shoreline the Converter supplies power and usually charges the battery but if it is shot it may not take a charge.
H/R Endeavor 2008
Ford F150 toad >Full Timers
Certified Senior Electronic Technician, Telecommunications Engineer, Telecommunications repair Service Center Owner, Original owner HR 2008