I wouldn’t use a vehicle ‘chassis’ as a ground point.
The battery cables to the block are probably heavy enough, but most -probably all- vehicles do not have a high amp negative(-) cable to the frame if there is a frame at all, AND the battery-to-body is designed to carry only the vehicle electrical needs at best. Most components are provided with hard-wired neg(-) connections and do not rely on the body sheet metal for a path to neg(-) except perhaps a single point near a PDC or fuse box.
Never minding that an LFP battery may not like the vehicle alternator’s charging characteristics anyway, the negative connection to the aux battery should be of sufficient capacity for the alternator’s amp potential. If your alternator is 100A, you should use pos(+) and neg(-) cables that can support 100A. If you’re using a DC2DC charger that is say 30A, you should use 30A cables.
I’d use the DC2DC charger with a lifepo battery. Smaller wire, potentially longer battery life
If you are not using a DC2DC I would absolutely use the vehicle body as ground, exactly because of the reason you are describing - it's a poor conductor.
LifePO4 batteries have incredible low internal resistance - something alternator doesn't know how to deal with and potential overheat.
That problem is easy cured with adding a little bit of resistance to the circuit. Hence including the vehicle body.
Do not ever connect a Lithium battery alone to a alternator. Always parallel a Lead battery and go through the vehicle frame.
The vehicle body is not floating ground in a charging system - that's only interesting for communication (data) and audio circuits.
Galvanic corrosion a thing of the past and that is not happening anymore. (at the terminals, yes but not on the frame)
Plan to use alternator as an option to charge house battery. Will either use DC-DC charger or VBS (voltage sensitivity relay) to isolate starter battery and house battery.
I'm using the Relay option. Measure !! that your alternator is not putting out more voltage then your chosen battery BMS can take in. Some trucks put out 15.5V and some are capped at 14.4V (ideal for lithium with a charge cutoff of like 14.8V )
Get a clamp on amp meter when put the system together - start with the lithium charged at like 80% before you actuate the relay the first time. - Measure the amps.
Slowly drain the battery and measure the amps flowing at various states of charge. (and engine revs)
Compare those values with your alternator rating.
If you get within 80% of the maximum of the alternator for prolonged periods (15-20min) - get a larger alternator. Still cheaper then a large DC2DC charger