You can get around this a bit, possibly, by using the battery boost switch to supply power to the chassis battery if you are concerned about running down the chassis battery and/or get a volt meter to plug into a lighter plug to periodically check on the state of charge of the chassis battery,
Two things to keep in mind.
If you use the radio or not after a while if you are parked in a campground for several weeks your engine battery voltage will be dropping and needs to be monitored.
You need a digital voltmeter as discussed. The normal voltage is 12.7 and never let it drop below 12.1 volts.
You have several options Start your engine before it gets too low and recharge from alternator., charge with battery charge as Ron recommends or the very easiest way is to just turn on the little switch just left of steering wheel.
What your switch is a momentary and must be held on? Just change it to an on-off switch that you can buy in an auto parts store and is a direct replacement, the wires just slip on and off.
I turn this on after I'm parked for a about a week and leave it on for a few hours.
Or turn it on while you are using the radio if you are concerned.