The voltage drop across the diode is on the order of 0.7vdc.  If the
alternator is measuring at the alternator rather than the battery the drop
in charging voltage at the battery will be at least 0.7vdc.  So if you
require 13.7 to properly charge the battery and the alternator clamps at
13.7 the battery will see 13.0 if the diode is in the circuit.  TANSTAAFL on
this one.  If the battery needs 14.1 then you are out of luck.  Only
modifying the voltage regulation system will change the charging
characteristics.  This of course assumes NO voltage drop due to corrosion,
long wire runs or poor connections at the battery.  All these factors
contribute to even a poorer charging environment.

Check the voltage at the alternator over a small range of RPM to determine
what the regulator clamp voltage is.  Then check at the battery terminals
themselves under the same RPM range to verify the battery voltage.  If it's
lower than spec for that particular battery then your charging time will be
long and perhaps never achieve a full charge.  Use a good meter.  Triplett
310C is fine as an analog meter most digitals over 40US$ are OK.  The others
are, well, questionable.

Best to measure rather than surmise.  Ebers and Moll pretty much decided
what the voltage drop across various junctions (silicon included) was.  A
good number is 0.7vdc.  Do the measurements on your van add 'em up and
you'll know.

"If one professes knowledge but cannot express that knowledge in numbers,
then his knowledge is of a meagre and insufficient kind"  Lord Kelvin

p