Yeah...I've done it...generally the shunt is mounted VERY close to the battery...within a couple of feet. Make sure that there is only ONE black wire left on your battery bank....the short one you buy to attach the shunt. all other negative wires should go to the other end of the shunt
or you'll get bad readings. If you have too many negative wires to all fit on the end of the shunt...you can make another short jumper cable up and attach it to a bus bar that has room for the connections and then to the shunt. Like this:
It should be kept dry but any kind of reasonable isolation from getting wet under the trailer should work fine as long as you inspect it when you water your batteries. If you're really worried about the location you can tape a baggie or wire loom over it.
If your batteries are in a plastic box...you could take a small square of plywood and mount an electrical waterproof/resistant junction box from home depot to it and place the shunt & connections inside then bolt it to the battery box tapoing and using silicone sealant.