How can I make my TT lights flash unhooked from my truck?

Try this.
Go to the auto parts store and ask a counter guy or get on Amazon.com and order an EL12 flasher unit.
E12 flasher unit

Now drive to the trailer and plug the trailer into the wall outlet or the generator.

Plug the flasher unit into slots 4 and 3 on the trailer plug.

See if the lights flash.

Unplug the flasher unit to stop the lights flashing.

If the lights don't flash you need to find a way to hook a fuse and 12 volt battery positive to pin 4 of the trailer plug.
Ground the trailer back to the negative of the battery.

Plug your E12 flasher in again and see if the lights flash.

It really is that simple.