Best practise for constant use is to get a deep cycle battery and run the fridge and radio from this with a dual battery isolator.
I realise you do not have one of these but it could be on the books for later.
What you could do is check out
this nifty little devise - it seems to work well and has an alarm that sounds for a minute when the battery voltage becomes low. It also incorporates a voltage display. I've found it very handy in the couple of hours of driving that I've done with it.
Ruby Scoo has a Supercharge battery, same sort of size but has something like 550CCA

I've got half of my dual battery setup installed, just need to add the switch and find an appropriate sized deep cycle battery, but they're rather expensive!
Cheers
Bennie