Probably the absolute cheapest way to get alternator charging (that will actually charge the LFP to full and not just to alternator voltage) is to get a 12 to 24v step up converter slightly bigger than your MPPT
(example), wire it to a source that is only active with the key on or engine running, and feed the 24v output to your MPPT in parallel with the solar panels. Any time the solar panel is exceeding 24v the converter will be ignored, but when the solar is making less than 24v the MPPT will treat the converter as the solar panel and charge from that.
