I agree with most of this mornings messages, but feel that some of the nuances of your installation are not addressed, for example I suspect you would be ok with just your existing charge line from your tow vehicle without the need for a DC-DC charger, though perhaps at less than optimal performance. Let me follow that to say that I am a strong advocate for the use of DC-DC chargers in Vans and motorhomes where the LiFePo4 battery bank is located relatively near the alternator connected with relatively heavy gauge wire. In these situations the charge acceptance rate of the LiFePo4 battery can overload the alternator, this is something that is much less likely to happen with a travel trailer due to the long somewhat smaller chargeline makeing it somewhat self regulating due to voltage drop over distance for any given gauge wire.
As to the mantra of 100AH of LiFePo4 replacing 200AH of lead acid, there is some truth in this for large stationary power banks designed for maximum battery life, this is often not the case in RV applications where batteries typically don't see this day after day, year after year cycling . For typical RV use cases I would tend to say it is more like 160A-180AHH LiFePo4 replacing a 200AH lead acid as RV lead acid batteries are often discharged below the point for optimal service life due to practical real world situations.
p.s. I have used a couple of those $20 DC-DC converters, they do not take the place of a proper DC-DC charger.
As to the mantra of 100AH of LiFePo4 replacing 200AH of lead acid, there is some truth in this for large stationary power banks designed for maximum battery life, this is often not the case in RV applications where batteries typically don't see this day after day, year after year cycling . For typical RV use cases I would tend to say it is more like 160A-180AHH LiFePo4 replacing a 200AH lead acid as RV lead acid batteries are often discharged below the point for optimal service life due to practical real world situations.
p.s. I have used a couple of those $20 DC-DC converters, they do not take the place of a proper DC-DC charger.