Load Distribution Block, Grounding and Enclosures

I am trying to understand your questions better by visualization. I am imagining a pop-up travel trailer with a 100 watt PV module that is deployable or maybe fixed. This in turn recharges a deep cycle battery utilizing some sort of charge control. Which in turn provides power for lighting and stuff by way of a DC rated, fused, power distribution panel.

At what wattage must the load go from 'wired to controller' to 'four terminal load distribution block' for an off grid 12v system?  We noted that the 50 watt did not require this but the 115 watt did.

Nope sorry, don't have a clue what your asking in the first question. My deductive reasoning skills seem to have eluded me there. I need something more.
Name brands, model numbers, something.

In building our mobile solar system at 100 Watts, as this unit is mobile, do we use a grounding rod or how?

Most low voltage, DC, mobile applications, off grid I know of do not require a grounding rod in Earth.

For enclosures, can fiberglass or plastic be used instead of metal?

I don't see any reason why it can't be fiberglass instead of metal.

Something I would like to point out is that most automotive electrical devices, such as but not limited to fuses and fused distribution panels are not rated for the higher open circuit voltages that a PV module can produce.