When enabled, RPG-HUD hides the tank HUD. This is a bit of an issue, because you can't see how much liquid is in the tanks, which tank is currently selected and what order the tools that you can put in the quickswap slots are in. 1.12.2 isn't supported by Traveler's backpack anymore, so that leaves only one way to fix the bug.

Traveler's Backpack 1.12.2-1.0.35
RPG-HUD 1.12.2-3.8.1