13.98 (Engine Running) with NO loads, is a little low.
It will however Maintain a good battery.

12.7(With only the headlights on) is not enough to charge the battery.

(Even though we call it a 12 volt system,you've gotta have at least 13 V
to even maintain a fully charged battery,with no loads)

I'd check the charging system wiring and grounds.

[Example: I've got a domestic that's showing 13.85 V @ Idle (No Loads) 128 K.
It's manual says to replace the VR if it's not putting out 14. V to 14.5 V.
'Ordered the new VR last week,after checking everything else.]

[Yamaha shows 14.41 V @ Idle OR @ WOT, with or without EVERYTHING including the Bilge Pumps,Trim Tabs and Engine Tilt (80 Amps draw) ON.]

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'84 300SD sold
124.128