[BENT_8]

Ive had my 135ah agm in an Arkpak for 5yrs now and its still going strong.
My best mate has the same setup, same fridge but smaller 100ah battery and is on his 2nd which is also giving grief.

Deep cycle batteries are expensive so people tend to buy to budget at the expense of usability, before buying a battery you need to consider load demand at its highest, a 50l fridge being used for food and opened 2 or 3 times a day in mild conditions will not draw too much so you shouldnt get too low, open it every half an hour for drinks and it draws more, thats ok if you have a big battery but if its borderline you'll have trouble.
The next thing to consider is recharge capacity, its better to have too much solar generation than not enough, however, the same principal applies, bigger=dearer and bulkier, unless you go solar blanket which is lighter but dearer again.
You get what you pay for as with most things.
Those cheaper fridges often use old tech inefficient compressors which use more power, they also dont insulate as well as some of the big brands.

The way i understand it a typical 50l fridge draws around 4amp, your 75ah battery is good for 35ah to keep above 50%, 35÷4 =9ish hours.
When camping you might have 12+ non productive hours overnight so already your behind the 8 ball, it wont matter how big your solar array is, youve dropped below the ideal voltage level before it came into play.
Start using that same battery for lighting or charging gadgets and the gap gets bigger.

It was explained to me that people often get the formula wrong, they either have a battery undersized and bank on bringinging it back up with bigger charging capacity or they have banks of batteries which are unnecessarily heavy and space consuming, the key is to work out your maximum power demands of all appliances combined, work out the total ah draw in non productive hours and buy a battery to suit with some to spare.
Using mine as an example, my fridge draws 4ah, right now in Adelaide were seeing roughly 12 non productive hours ie. 8pm to 8am, 12×4=48ah of draw, from my 135ah battery i stay above 65%, the same applied to your 75ah battery see's you at 35%, too low and hence the low voltage trigger.

My advice with what you have would be to limit access to the fridge after sunset and when u can, get a bigger battery.
In the meantime, invest in a multi stage charger and keep it on float cycle at home.