Battery Meter

I use a digital voltmeter that I plug into a cigarette style outlet that I installed just inside the door of the TT. I can also use that socket to charge cameras, flashlights, anything that needs a 12 volt outlet. Cost me about $15 total and multipurpose.

Victron Battery Monitor. Several different models, some with Bluetooth. Not cheap, but probably the best one out there.

Basic LCD meter will use less power than the battery self discharges.

Lots of digital panel meters are available at Amazon, etc. One of those plus a switch would work (though without the needle, of course).

Here is a nice looking analog gauge for battery voltage, with an appropriately sensitive range. I don't think it would be great for your purposes, though, in as much as any converter would peg it on the high side. When operating off of battery power it would be fine; maybe have a momentary pushbutton switch to connect it in under those circumstances. (No numbers on the scale, unfortunately.)

I have installed a BM2 from ebay. No local display but it is bluetooth to your phone. You can check you voltage from bed and it has data logging that allows you to see your voltage on a graph.

This past weekend I knew from bed that my batteries had died at 3 am for some reason. That reason turned out to be low water.

Look at the AiLi monitor on Amazon

There are all sorts of things floating around eBay. I believe I'm going to use one of the victron bmv series. But I'm here for better suggestions as well..