12v fridge combination of size dilema

You already have the perfect combination.
Keep all the drinks and veg and dinner stuff in the Jayco, and put all the lunch stuff and essentials such as meat and milk in the Engel in the car.

I used to load a heap of beer and the wifes tins (as much as I could cram in with the food) in the fridge for a trip, but now do it differently.
Each morning, I just put say 4 or 5 beers and a few UDL's in the fridge for that late arvo / night, by days end they are nice and cold :)
Driving the car during the day, I just run the fridge from the car battery, as it's being fully charged anyway, at night plug it into Arkpak 100 amp/hr agm.

We use a 45lt size as a fridge, set at -2, it runs between -4 and 0, and find it great for food and drinks stocked as above for 2 - 3 weeks.
When we start doing longer tours, I will likely get a small 2nd fridge / freezer (15 - 20lt) that can be used as a freezer for meats, meals etc, and I reckon we will have a perfect setup for trips of 4 - 6 weeks before stocking up.

Put you tins (bottles too) into size 4 kids socks, stops rub throughs and check at Kmart etc.
We call them candoms :)

Use the fridge as a fridge, the 32 litre engel as a freezer and buy a 38 lt engel for the car for drinks and snacks. (or vica versa).

Definitely 2 fridges, gives you lots of flexibility and options on different trips.