I’m assuming the fridge is a compressor-type fridge and not an absorption fridge.
A compressor fridge normally draws about 3 Amps to run the compressor. Normally here means any time except startup, when the current draw leaps up to about 30A for a small part of a second. That’s because the motor draws a huge current when it is standing still, but that current drops very quickly when the motor starts to turn. That’s why it has such thick power wires.
Because the startup current, though high, is of a very short duration, and because the fuse reacts to heat over a longer period of time the fuse does not need to be rated for the full startup current. However if the power supply wires are thin they will inhibit the high current at startup which will affect the fridge’s electronic controls.
So, the 15A fuse makes sense to me, although it is a lot higher than you need to run the fridge under normal conditions. Be sure to supply the fridge with at least 4sq/mm wire.
If you have an absorption fridge you will have a much higher normal current because the fridge works with a heater which needs a lot of power. The heater boils amonia gas out of its solution in water into a pressurised hydrogen atmosphere and needs to be above 60W to function. Domestic absorption fridges have heaters that need 300W of continuous input energy, but caravan fridges are a lot smaller of course. So because there’s no motor you don’t get the startup surge current, but you do need to supply more than 5A (5A x 12V = 60W) on a continuous basis.