Hi Jam49,
My answers to:
1. Yes, use with any 12 volt battery. Can revive most dead batteries as long as there isn't a mechanical problem such as shorted plates. They reverse sufation and turn it into usable electrolite. New batteries begin sulfation immediately and we are constantly charging and discharging when the trailer is in use and when it sits idle. There are different models. Some sit on or near the battery and use the juice to keep the pulse going, stopping when it gets lower than a certain voltage. Others only work when the battery is being charged by the alternator/hook up/genset etc. Then there are those which have a trickle charge to maintain sitting ones.
2. You can install. They're really, Really simple.
3. There are many brand and types: it depends on the need and equipment. I got a solar powered one with a 110 power source. It can be used either way. They come in 2-5-10amps. Take your pick. Higher power ones work more quickly. It does take time to restore. Since my last post my batteries have reached 12.7 Volts resting. This is the normal max charge for Exides from their website.
Pulse Tech makes a whack of different ones including the Solargizer line. Pulsetch.com I got mine from a Canadian outfit to avoid customs/duty and shipping challenges.
Infinitum is another and attaches directly to the terminals running full time.
4.I have 5 12 volt automotive batteries. 2 in the oliver about $5-600 to replace with quality deep cycle high amp hour. 1 Smart car,expensive german unit, I don't want to know! 1 motorcycle at around $100 and 1 for the truck, God knows..... Anyway it's a real saving to be able to extend the useful life. I use the Solargizer on all of them. It had clips. I could get 3 more and keep them hooked up continually but it has to stop somewhere. Gives me something to putter and fiddle with.
There are CO and gas detectors plus the inverter that I know about. If the fridge is on auto, after AC, default is DC. Propane is cheaper and uses much less power but that has to be chosen from the menu each time we disconnect. We never turn the fridge off, ever. Done it like that for years and it runs fine.When I turn my stereo off on the main bar by the door,the blue light extinguishes. Do that nightly.
Oh incidentally smaller and lighter means less juice shorter use. You want the heaviest lead filled sucker you can find to fit that box. The OEM's are top of the line 12 volt. Blue top Optima would be better in that they don't leak at all. Big $'s.
On the 6volt vs 12 volt it's most likely cost issue. HQ 6 volts are more money. I'll replace mine next time, 10 years from now.... with 6's.
Hopeful this is helpful.