Split LiFePO4 "coach" battery from Chevy "engine" battery?

[rickst29]

Hi. I'm assisting the owner of a 2010-vintage Class B, gasoline powered on a Chevy Van chassis. We're upgrading it with LFP batteries in the rear. I'm a bit concerned that they will tend be "drained down" by a lower voltage SLA/AGM "Chevy" battery under the hood, unless I put in some sort of separation scheme. My first idea is to put in a DC-->DC 20A Renogy "Boost then regulate downwards", with Lithium charge settings. But I worry slightly about something:

With the interconnect being only 20A maximum, I must hook up the starter of the Onan Generator to one side *or* the other. Pulling considerable cranking amps, I'm inclined to wire it into the "Chevy" side exclusively. (If the "starting battery" and alternator can't generate enough Amps for the Onan, while the "Chevy" engine is running, then I think I have a battery problem in the Chevy - not a need to steal power form the Lithium. The Lithium can do 200A for 10 seconds, but not more than that. I SWAG that the Onan might want more in cold weather or high alititude.)

What do you all think about this?

[pasusan]

^
Why... Hanging underneath, of course.

(Makes it oh so easy to work on.)

OP - Can't help, but I think most if not all class Bs have a battery isolator that allows the coach batteries to be charged with the engine alternator, but doesn't allow the engine battery to draw from the coach batteries.

[theoldwizard1]

rickst29 wrote:
Pulling considerable cranking amps, I'm inclined to wire it into the "Chevy" side exclusively.

Concur !

Segue - Where on a class B is there room for a Onan generator !

[pianotuna]

Instead of guessing, measure the start up amperage of the Onan.

I'm leaning to SiO2 because of cold--and also they can do 900 amps from a 100 amp-hour battery.