Do not try to put the ammeter shunt in the positive battery lead, or you will exceed the common-mode input spec on your DPM used for the ammeter. It only allows a common-mode voltage range of +-1V.
You can put the shunt in the battery negative lead if you can disconnect all of the loads in the trailer from frame ground (unlikely). See the attached diagram. It shows the voltage divider for the voltmeter, and the method of connecting the shunt. This will not work if you cannot isolate all the loads.
You will need an LM7805 to make the 5V to power the DPMs.
btw- I did this in my trailer, but I used LCD DPMs. They draw so little power that I leave them on without a power switch. Your LED DPM will have to be switched off so that you dont kill the battery....
A much better way of doing this is to use a high-side-current-monitor (
Like the ZXCT1009). This lets you put the shunt in the battery positive lead, and tailor the gain so that you could get 20A full-scale instead of the 200A you will naturally get with the +-200mV DPM. A better choice for the shunt would be 10mΩ (10mV/1A).