doin the maths- engel power consumption

Hi all, doing some research before replacing our Trojan marine battery in the camper. I was doing some loose mental math as to my power usage on the engel but got a bit lost trying to workout if there is a prediction on how long it cycles for at the different dial settings. I started out using maximum 4.2 amps and cycling 50%, however on reflection- with the fridge (its a 57L combo) running on no 5 and keeping the freezer nice and frozen, it would problably be running close to 100%. Short of sitting there measuring each time it cycles on and off is there a better way of doing my calculations.The Troj was down to 12v(under load) after 12 houirs which is the worst it has performed. It is now 5 years old and has been mistreated terribly in the past. I was hoping to replace it with another battery and get about 48 hours before having to recharge. My caclulations suggest a 130ah unit should achieve this however want to be more sure of the numbers before making a purchase. Also is it true that Engels don't have a low voltage cutout and will suck a battery dry?ta, doc.
(ps -I know this falls under the dreaded battery/12 volts suject heading- play nicely everyone!)