Electronic – Calculating total resistance between two points (triangle resistor network)

If you're looking for Rth, you do exactly as you said (remove RL such that there is an open-circuit between A and B, then shut V2 off and measure equivalent resistance from A to B).

First, remove the resistor and short V2. If you label nodes at this point, it will help you to redraw the circuit in a more typical fashion:

Convince yourself that that is true, and from there you should be golden :)

Try re-drawing the schematic:

^{simulate this circuit – Schematic created using CircuitLab}

Hopefully the solution will be obvious once you do so (comment/ask if it's not).