My betting is you've one bad
battery at least out of your leisure set...
I have the same problem jump lead wise with our Ducato....
Positive is right under the wing/bulkhead (and its a pain because it has a multiple connector plate too)
I bought one of these and a suitable positive lead to connect it...
Longacre battery jumper posts are an easy, innovative addition to any battery wiring circuit on cars where the battery is hidden away under the seat/in the boot etc, or where access is really quite tight. Available in positive or negative options (colour coded caps included), they can be easily...
www.demon-tweeks.com
I've still not got around to fitting it though lol.
As above I'd get a
battery charger on the starter
battery THEN have a check of your leisure ones.