[EC900]

With no cranking issues to start with, thats still in the good range at +13v running, but +14v is likely with a new battery. Cable connections kept tight and clean.
Before jumping into alternator and regulator, better measure would be to get a cold reading under load cranking/ starting…+9.5v to see if battery is on its way out.

Even using a tender the battery is still going to age out. Some last 5-7 years, some only 2-4 yrs. The minimum to start an engine might be about 12.4v with accessories off.
I believe a battery maintainer will hold the battery in a range of 12.7 to 13v.