The actual current needed to crank the engine varies considerably, depending upon whether it's a little 4-cyl motor or a big V8 and whether it's hot or cold etc. Diesels also take more cranking due to the higher compression ratios.
I once bought a camper van which had the original vehicle battery slung under the chassis, truck style (it was based on the Bedford CF). Somebody had wired an auxiliary leisure battery and just kind of strapped it into place behind the driver's seat. The wiring was just a shabby, with no fuses etc. I soon moved that onto the tray alongside the original battery.
Apart from anything else, one of gasses given off during charging is hydrogen. Not exactly what you want floating around inside a camper where you have naked flames on the stove!
