"At a glance" is what I call this. While driving you can see this through your rear window and tell what the state of your battery voltage is onboard the vehicle you're towing. This is assuming you have a battery onboard that towed vehicle! So you know at a glance the voltage.
This is very simple to connect to your plug harness distribution box on your towed vehicle. I use this on my 5th wheel, and it works perfectly. As seen in one of these photos, it's above the pin box, but below the fiberglass nose, so it is quite visible when connected to your hitch. It is waterproof and easily visible.
Please check with your manufacturer to know which lugs to attach the leads from this to. If you have a voltmeter, it's simple as well. I can't presume the wiring of every trailer, so please verify yours. This reads voltages from 4 to 48. Most towed RV's are putting out 12v, even if you have multiple batteries installed.
Instead of wondering if your battery is holding a charge after all that driving, you can just glance at the meter and know right away!