For a battery monitor to work properly it has to be installed as you have found out with the chassis ground connected to the system side of the shunt, See the FAQ linked below. This is the only way for it to work, all current in and out via the negative cable is counted. Anything else will not work. I think you now understand this but I am linking it anyway.
Shunt wiring as a battery monitor
If the easy solar is set as a battery monitor then it does not know anything about your DC loads so it will be less accurate. If you set the SmartShunt up as a DC meter then this will measure the DC loads but I do not think that this gets transmitted to the EasySolar for it to add this into the consumption.
I think that you are going to have to either accept something less than 100% accurate or swap the chassis ground wire to the SmartShunt.