Hey
@DRW the link that
@Glen Bundesen noted gives a pretty plausable reason I thought, though how it goes in real world may be academic like a lot of electrical bench findings.....I wired mine up like that so it must be right...lol lol,lol.
In a perfect world
1. the original battery is probably down on capacity so both batteries should be replaced with identical batteries
2. the joining cables should be large enough to prevent any voltage drop for any type loads, think heavy battery cables
3. the van would have a state of the art multi stage battery charger and multi stage DC to DC charger
4. the solar charger will also have multi stage charging to ensure the batteries are fully charged
Now what we really have
1. a used chinese battery so adding a similar battery will usually double capacity
2. a battery charger that is a trickle charger that it set to an average voltage to charge the battery to an acceptable charge for running what's intended
3. factory wiring (no further comment needed)
4. a van that uses bugger all power in most cases, mabe a TV, a few lights and charges electronics etc possibly a 12V car fridge from time to time (as I said if you are running high current loads the whole thing needs to be re-hashed)
At the end of the day the wiring modifications are usually being performed by a novice (no offence intended) usually in a 17' there is room to fit a battery next to the original battery, the original cables wont always reach the second battery and joining them is less than ideal. Using 6mm auto to connect the battery is using the same cabled Jayco use and with the standard electrical usage there will be nothing in the way of voltage drop.
In my last 2 Jayco's I threw the original charging in the bin, fitted a decent charger, a decent DC to DC charger and had 120W portable solar panel (I often park in the shade) I almost always had my 12V waeco fridge running off the van as well, and in both vans we only had the original battery and all worked fine