30A to 40A trailer fuse

op is gone

Every fuse location on my 2007,2013 and 2018 had the fuse amperage at the installation point.

You can also use a cartridge fuse referred to as a 'slow-blow' Instead of having clear glass slow blow fuse centers are white. Gives you the accurate protection of 30-amp fusing but will withstand the extra amperage that a compressor uses for start up.

Always fuse a general feed circuit for the wire amp capacity. Fuse branch circuits for the load. Fuses for anything electronic and circuit breakers for anything else.

If the owners manual says that slot takes a 40 amp fuse then by all means put in a 40 amp fuse.

If I understand you correctly your truck has a 30 amp fuse however the manual/specs. call for it to be a 40 amp fuse? Are you the original owner of the truck?
If the fuse has been changed from the factory spec of 40 amp to a 30 amp I would certainly change it back to a 40 amp.

You can change it to a 40A if that is what GM designed it for. It's possible someone previously swapped the 40A for a 30A because it was easier to find. I would look in the truck's owner's manual and see what size is supposed to be there. If it says 40, fine, put in a 40. If it says 30 then leave it alone.
I'm not surprised it's blowing. 30 amps is only about 360 watts. Your refrigerator likely pulls a good bit more than that. I would have thought the long run of small wire in the truck and trailer would have limited the current but maybe not.
Or, you could have an intermittent short in the wiring somewhere.
If you get a clamp-on amp meter that reads DC you can measure the current flow with the refrigerator running.
I take it you have a residential electric refrigerator? If your ref. will run on propane I'd just run it that way since trying to run it on 12v is pushing the limits of the circuit.

Reason #2 why I have an RV type Absorption Fridge

THe fuse is there to protect the wire IF you put in a BIGGER fuse the wire can overheat and .. FLAME ON.. You do not want this happening.

Ours came with a 40a fuse in Stud 1, but IMO that is not the problem. Never saw more than about 20a using the 7-pin, but had problems at times back at the 7-pin bumper end with corrosion shorts etc. A short back there or with the trailer's end of it is more likely to blow that fuse IMO.

The inverter should be on the house batteries with good fat wiring and get its amps for the fridge from that. The 7-pin just helps a bit.

I'm not sure I understand. Is this fuse for the wire supplying the trailer with power to charge batteries?