Robert,

the Load is a "Blackbox" that requires a stabilized 12V supply voltage. (max. 5A)

Power is provided by a solar panel and Lead batteries
(this is why the input voltage might be between 10 and 14 Volts)


There is no way to modify the "Blackbox" and tests have shown that it does not operate at a voltage lower than 11.5V.
The datasheet says the supply voltage must not be higher than 12.5 V.
I have not tested at what voltage higher than 12.5V it starts to generate smoke.

My initial thought was to use a StepUp converter in the first stage and then step down to 12V.

A second point is:
The environment temp will be up to 45 or even 50 deg C.
This requires the converter to work at high efficiency to reduce thermal losses.

Do you have any hints ?

rgds