qldit

Good Afternoon Kal061506, it is possible there is a problem somewhere between the actual battery and the blower.

The in cab voltmeter does not read the voltage directly from the battery but picks up usually somewhere like the incab fuse box.

Now if there were a defective connection enroute from the battery this could cause your problem.

I would suggest having the vehicle stationary and having someone monitor the incab voltmeter during the problem being apparent and at the same time have a separate suitable meter on the actual battery posts to see if that voltage reading is in sympathy.

If only the incab voltage is dropping, examine all connections by feel, chances are you will find a "hot" connection somewhere (like on the back of the incab fuse block or under the hood at some point, possibly in the area of the underhood fuse panel if it has one. If the vehicle has fusable links near the battery area, doublecheck them for generated heat.
(Voltage drop across a connection develops heat)

If the voltage at both places is dropping similarly it would suggest the alternator may not be producing enough current.

Voltage readings and "feel" are usually the key in these kind of problem analysyis cases.

Ensure the battery is receiving voltage in the order of 14 volts or better when running the engine at a stiff idle with no electrical loads applied. (14.4 volts is expected at more or less maximum)

Ensure there is no voltage drop between the engine and the chassis with all electrical gadgets running.

Others will have different ideas

Cheers, qldit.