Don't victron have a reccomended size for this cable? My Renogy (which is a 50A charger) manual did have, 16mm from the van battery to the dc/dc. So 10mm sounds ok for 30A. It's all about what volt drop you can tolerate really, as even 4.0mm cable can carry 30A but volt drop would be terrible.
For 10mm cable volt drop is 1.9mv/A/M and for 16mm cable volt drop is 1.2mV/A/M. So for a 5m run/return at 30A volt drop is 180mV for 16mm and 285mV for 10mm.
You'll need to work it out for your install and decide for yourself........ or just fit 16mm!
For short runs, i.e. 0.5m from victron to battery under the seat, I wouldn't stress about 10mm vs 16mm volt drop, so long as the cable is rated higher than the fuses.
Another thing to note; cable ratings are ALWAYS dependant on installation type, if it's in free air the rating will be much higher than if it's bunched with other cables and buried under a carpet. Make sure you use the rating that applies for your install.
NOTE:
I've read on the internet (so it must be true) that the van body (given good earth bolts) is equivalent to an extremely large copper conductor due to it's effective CSA. So in actual fact volt drops will be less than those shown above.
