duel batter system question: DC/DC or Isolator

[Cuppa]

Thicker cable is the way to go. 1.5 metres is not much of an issue, but remember you have to double the distance to account for negative & positive. (Total length). Check out the table at the bottom of this page it gives recommendations for wire size to ensure minimal voltage drop for given current carrying capacity & cable length. You will see that for 3 metres with a 40amp charger the recommended cable size is 10mm2 (not including insulation as is the case with autocable). This equates to somewhere between B&S 6 & 8. Go to the next size up ie. 6 B&S & you'll be good.

In theory if you connected the battery under the the vehicle to the two in the camper trailer in parallel so that the charger saw them all as a single battery you would only need a single charger, but I think you would want to be using much heavier cable which could become both expensive & impractical. If you consider that many under bonnet dual battery systems have the batteries joined using 35mm2 cable for relatively short runs I think you would want to go much heavier to the camper trailer if setting up in this way.

Best to have a dc to dc charger in the camper trailer taken from the crank battery. Depending upon what your intended use of the under vehicle aux is, you might be ok with just an isolator solenoid eg. If using it when winching with the motor running. But if planning to have say, a fridge in the vehicle running off it I would say that you would need two dc to dc chargers.
You could have solenoid between crank & under vehicle, & then dc to dc between under vehicle & camper trailer batts, with the dc to dc mounted in the trailer, but depending upon the cut in & cut off thresholds of the solenoid, you could find that you were not getting as much to the trailer as you could with a direct connection between crank & trailer batts (via dc to dc in trailer) if you follow me.

For crank to trailer .... If you are buying a Redarc charger ask them for their recommendation on cable sizing, their technical service is excellent (a good reason for buying their gear). .... According to here they recommend 16mm2 between crank & BCDC1240 when it is is mounted in the rear of the vehicle. heavier again in trailer. 4B&S is about 21mm2. Welding cable is more economic

When working out how best to do my set up Redarc were great. I emailed them a copy of my proposed wiring diagram & they promptly got back to me with their comments.

Cuppa