Car/caravan split charging

[G Dog]

Hi everybody
This is my first time on this forum, so hello one and all. I'm a dab hand with soldering iron etc. but apart from very basic principles rather short on knowledge. I want to put together my own split charge relay system to use on my car and caravan, I know I can bye one at reasonable cost, but I fancy making one myself just for the hell of it. What I need is a diagram with all component values for a circuit that will give me an output at 13.4 volts and will be able to operate a relay coil of 77 ohms, (180ma more or less) and then switch off when the voltage falls to below about 12.8 volts. Not much to ask from enthusiasts with young brains I would have thought, mines getting on a bit so try and keep it simple. Thanks in anticipation.