13.9V is certainly well within the safe limit for charging and a simple test with a multimeter will confirm whether the Madman or NL gauge is accurate.
i would for peace of mind just have someone confirm the condition of the battery before setting off on the trip. i know the batteries are new but one can never be too sure. if faulty, they can swop it out under warranty before hitting the road.
you have had so many issues with the alternator/s since getting the batteries replaced that one would never know if there could possibly be residual damage.