In order to accurately monitor 100% of the load, the shunt must be installed between the the negative battery post(s) and all negative returns from all connected devices. AS typically wires the tongue jack and the inverter using separate positive and negative leads. So the best setup is to re-wire in the battery box a bit. You most likely will have to use an increased wire size from the battery post to the shunt and the size would depend on your inverter size and the combined main fuse sizes for all other loads. You do not need to increase the size of the wire connecting any batteries in parallel.
If you have your heart set on placing your shunt in the area pictured rewiring become just a bit more extensive. You will have to extend your jack negative and all other negatives (solar charger, external charger leads, added aftermarket stuff, inverter, etc.) into the area pictured. then up-size the black negative battery lead connected to the white "neutral/ground bus" to a #2 (or larger) connect it to the battery side of the shunt and connect all other device negatives to the pictured ground bus (if it has room, if not get a larger bus bar) then connect another #2 (or larger) between the ground bus bar and the load side of the shunt. Easy as that....