To be honest with all of the messing about wiring etc. I would go wireless
https://www.manomano.co.uk/catalogue/p/doorbell-door-viewer-camera-door-peephole-door-camera-doorbell-with-wireless-monitor-56693351...
Do you live near a transmitter?
https://www.allaboutcircuits.com/technical-articles/wireless-rf-energy-harvesting-rf-to-dc-conversion-powercast-hardware/
Use the old builders trick with pricing.
Look around, scratch your head and draw breath through your teeth causing instant alarm to the customer, whilst making out it is the worst job you have ever come across.
Then say it's going to be 3 grand but you'll do it half price for cash, making sure...
Lead batteries/gel and the likes of shouldn't be run under a certain % of their capacity, solar and a Ctek250se will keep you going all day and night, though you will have to start the van periodically through the night for dc charge.
A shunt will keep you clued up to what's going out and...
Before phones came along everything seemed to flow better.
True to say that if you broke down you might find yourself using up energy and causing yourself to become fitter by walking 5 miles with a petrol can, but messages still got around it's just they didn't feel so important like they do...
I remember years back in the mess hut, a Carpenter was telling a Bricklayer how he kept a Rockweiler in the car for added security.
In which the Bricklayer replied, 'so it can put out fires can it'
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