Im thinking low side restriction
but the batterys are first issue, need solid CCA before anything else can be diagnosed
are your batterys in good state of health and have a parasitic draw or are they just bad?
one bad battery can take out the other so test them both carbon pile test or a load tester w/e is available to you
min cranking psi should be 7psi , clean filter and lines that are not kinked or restricted
does the engine run? after boosted?
with what you have said so far this sounds like a low side supply issue