I got something like this years ago from Radio Shack:

https://www.amazon.com/VIGORWORK-Thermometer-Voltmeter-Temperature-Backlight/dp/B077XH9L6H/ref=sr_1_11

It just has 2 displays that you can switch to a variety of things...
indoor temp, outdoor temp, voltage, clock, etc.  I always kept it on
Voltage and outdoor temp.  The voltage input wire I ran through a toggle
switch as you suggest so it could display either battery's voltage.  the
sensor for outdoor temp I ran to inside the fridge so I could monitor
that.  I have found it pretty handy to have that info available.

Edward

On Wed, Feb 27, 2019 at 1:08 PM Edward V Barger <edvbarger@mac.com> wrote:

> I am slowly making progress on the 87 Westy   Looking at this or something
> like it to monitor both my batteries
>
> Thought I could put in an A/B switch so could toggle between the two to
> check levels
>
>
>
> https://www.amazon.com/gp/product/B07759SLYP?ref=em_1p_1_ti&ref_=pe_3743280_396929690
>
>
>
> Edward V Barger
> edvbarger@mac.com
> 310 922 6251
>