100 s TD and 60L Engel Fridge

your 60L Engel will draw more than most fridges but if you turn it down of a nighttime then I'm sure you'll manage fine for 1 & even 2 night stays. Recharging might be a problem with a parallel system though. Perhaps you should look at a smart solenoid or similar controlling system(search on this forum should give you all the info you need). I have a Rotronics RDC12M but that will set you back about the $800 you were talking about originally. Rotronics and indeed Piranha have cheaper alternatives.

I meant to say "turn the fridge down to minimum at night"

the Waeco Blue Apple range of batteries is excellent - especially the Thumper heavy duty industrial, I have one. But if you buy one of those you won't get much change out of $700 and I think that Mork is looking for a less expensive solution

Yeah the thumper is nearly $700, but that's also for jump starting. The basic battery pack (for running the fridge only) is $299 and would easily run the fridge overnight. The fridge could be run from the car batteries while driving.