another DIY roadtrek lithium - newb needs help

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I have a '15 Roadtrek TS. It has a second alternator charging 4 coach agm batteries and connects to a renogy 3000w inverter. All 4 of my agm were on their way out, holding very little charge so I thought it was a good time to start converting to lifepo4.

I purchased a single battleborn GC3 (280Ah) battery which will fit in the rear battery tray, replacing two of the original agms. Originally the plan was to replace the two under hood 6v agm batteries with a single 12v automotive battery that will then connect to two sterling 60amp DC to DC chargers and then connect the existing 4awg cable that goes from hood to rear battery. Is it a bad idea to install the sterling chargers under hood where there will be a lot of heat?

After reading more threads in here some people are for the dc to dc charging and some are about installing a ballmer. When I started my research I shied away from ballmer because I heard those sometimes fail causing battery issues. Is this an overblown concern and therefore should I be leaning more toward the ballmer straight to lithium skipping dc to dc?