15 amp fuse keeps blowing when hooked to generator

I don't know why this has to be a guessing game, you have a newer trailer. A call to Keystone, a look through the owner's manual, or a call to the dealer would tell you what this fuse protects and if it is of the correct size.

A single 15 amp fuse would not be big enough to protect your entire 12V system. It protects a circuit that is part of the 12V system or is not correct for the circuit. If anyone tells you to just replace it with a larger fuse, that would be dangerous advice.