Why won't my aux battery charge from shoreline/the alternator?

Hello, my name is Aten. I own a 1995 Eurovan Winnebago full camper (with the pop top
and kitchen, etc). Vin #VW2EE0703SH034694.

I am living in Chiapas, Mexico at the moment, and cannot locate anyone who can help
me with the van. (I’m here studying Spanish, but as you can imagine problems with the
van are too complicated for my limited linguistics. In addition, this car is extremely rare
here as are marine batteries in the mountains of Chiapas).

So, I’m writing to explain my problem and see if you might be able to direct me to
someone that feels that they understand how to fix the problem and can walk me
through on the phone (or via video Skype), I am happy to pay for the time. If they can
answer via E-mail, all the better.

Here’s the problem:

I have an Interstate Deep Cycle Marine Battery. It’s about a year and a half old. I have
had it tested at a battery store in the US and at the local Autozone here. Both tests list
the battery in good condition.

I own the GoWesty Solar kit. It charges the battery when I plug up the solar panel.
When the sun is really hot, the batter goes to full charge 14.5V. Usually, it hovers
around 12.34V.

However, the battery does not charge at all from the alternator, nor when I use shore
line power (this shoreline problem was true before my arrival in Mexico as well, so I
know that it is not a problem with the Mexican electrical grid).

But, the battery powers everything fine (the fridge, the furnace, my inverter, the LP
alarm). The only things I haven’t tested are the 12V outlets because I never use them.

When I’m plugged into shoreline, all of the other systems (besides the battery charging)
work fine: the internal lights switch over, the internal electrical outlets become active,
etc).

My knowledge of cars is limited, but I do have two things in my favor: time, and the
complete service manuals (although, they are incredible dense). I live in my campervan,
so my need for a functioning system is a bit greater than someone who uses it in an
auxillary way.

Thank you in advance for any guidance you can provide.
Aten