If you're going to use a volt meter alone for the rear that's fine, but an ammeter won't really work.
Ammeters work by measuring the voltage drop across a precisely known resistance (called a shunt). ALL power flows through the shunt, and the more current (amps) that flows through, the greater the voltage drop across the shunt.
The problem isn't that it's inaccurate - it's actually quite good - the problem lies in the length of cable required to get the display (which usually contains the necessary circuitry) into the cabin. Even with decently heavy cabling, there's enough voltage drop over the distance to make the measurement of the voltage drop between the two sides of the shunt virtually impossible.
I've opted to do things a little differently in my car. I have two displays of auxiliary power.
1) Pure voltage for the battery in the tub. It's a good enough indicator. I have a solar panel up on top of the canopy anyway, and a solar regulator inside that maintains the Optima Yellow-top D31A. I also have an inverter that drives the fridge on 240V while the engine is running. The system is self-sufficient - it's only on days of excessive heat where I'm not driving that the solar system can't keep up with the power demands of the Engel.
2) Volts + amps - for ALL power heading to the trailer. I've got two power feeds (a total of 4 wires, because I run positive and negative) to allow a total of 112A continuous to the rear. One of these feeds through the 12pin trailer plug and supplies power to the breakaway battery and house battery charging. The other supplies power through a 50A Anderson plug to supply power to the fridge. The breakaway battery is negligble, ignore it. The house battery is kept topped up by the solar panel so ignore it too, meaning that the only power being sent to the rear is for the fridge. Since the shunt is sitting on top of my starter battery, the cables only have about 1.5m to run into the cabin - perfect.
