I climbed onto the bar work and had a look, I just can't find them.
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rofl........
I go with evil twin on this, especially the highlighted, have you tested yet?
You could take it back to Deerings and get them to check the whole system, as they did the work.
I know others with similar problems. When you say your batteries have CCA and are deep cycle then they must be, what they call dual purpose, and this IMHO is the problem (or part of).
We know we need CCA for a starting battery, and most folk have an auxiliary battery tray for auxiliaries, so a purpose built for each designation is the go, and a wet cell handles heat better under the bonnet.
The other problem that is servicing is sealed batteries, if they have a high heat or high charge situation they can't be maintained. Just another casualty in the 'lazy, throw away world' that it has become.
Other things to check over is the changes we make to our 4bys from OE. In this case the engine bay cooling. So a lift, spotlights, winch and other add-ons can effect this. Make sure the skirts from the inner guard to the chassis are in place and in good order. The heat is supposed to be carried away, tunnel like, under the fire-wall.
Good luck.