Will it hurt anything if I have a 12/12 18amp dc-dc charger installed if the trailer wiring will only support a max of 10 amp from the 7pin? 10AWG trailer wiring. Safety issue? I find the Victron smart 12/12 dc dc chargers start at 18 amps. My truck is a Tundra with a 30amp fuse for the trailer 7pin power.
Your 10-AWG "trailer wiring" can support a maximum of 30A, so that's not a problem. The "Trailer Battery Charge" wire within the Tundra is possibly smaller than the fuse rating, but should be OK - maybe causing moderate Voltage Drop when carrying all 18 amps. (In my own 4Runner, Toyota installed a smaller gauge TBC wire at the factory, and I later upgraded it to 10-AWG.)
Your highest risk of "burnout", when pulling all 18A into the DC/DC charger, will occur within the Bargman power cord - and especially at it's wire ends, connecting into the 7-pin terminal for the Tundra connection. But your DC->DC charger will probably not need all 18 amps. The Victron "smart" unit may also allow for tuning to a smaller power limit.
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In my own configuration, I boost the alternator/engine "under-the-hood" voltage from its original value, typically around 13.5 Volts, up to 36.0 volts for the Bargman TBC wire connector. (This is under control of dashboard switch, which allows me to leave the switch "OFF" when towing other Trailers.) In my Trailer, I use a 30A MPPT Solar Charge Controller to receive the high-voltage TBC input, converting down to LFP battery charging Voltage in accordance with battery state-of-charge.
The resulting current load (within my Bargeman Power Cord, The 4Runner TBC wire, and the Trailer-internal "run" to the Solar Charge Controller) is only about 12 Amps maximum. It offers maximum output of almost 30A, rather than only 18A. But the installation under the 4Runner hood was complicated - involving the new dashboard controller switch, a high-power Relay on the path between the Battery and the "Booster" unit, plus BIG wires (AWG-8) and a 40A fuse for the leads into the Booster.
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The advantage of my personal scheme is a 3x reduction in current on the TBC wire. The disadvantages are higher cost and very complex installation. I think that your use of the Victron would likely be problem-free, although it creates significantly less maximum power.