You could try popping in a 7.5 amp fuse. I'm not saying you should, but many systems are built with some excess capacity. Personally, I would be willing to try a 7.5A, and would consider a 10A fuse in a fuse socket rated for 5A, but would not be brave enough to go any higher.
Having said that, on my C14, I added 2 Powerlets wired hot to the battery and am happy with that. Another cheaper alternative would be to hot-wire an SAE/Battery Tender outlet to the battery and use that to run the compressor.