Hi Peter,

Have you had any odd noises and perhaps stiff steering along with the
poor charging?
You could also have an issue with the serpentine belt or failed belt tensioner.
The belt can get old, glazed, covered with oil, stretched, misaligned, etc.
Here's one of many write-ups:

https://www.samarins.com/glossary/serpentine_belt.html

Poor charging under heavy load is common with loose/glazed/misaligned vee-belts
Others might comment about problem S-belt symptoms.

I'd still look for a ratty cable crimp or alternator connector first.

Post back with your findings, Bill