My Gen.2 -Now only holds 31/2 gallons of gas in the tank bladder

All sorts of plumbing/pressure/vacuum action going on in this system. I guess it follows that it isn't fully instrumented electronically.

Consider the mechanism within a gas station nozzle. Nothing electronic in its operation, yet lots of intricate logic mediated with tubes, balls, cones, springs, and diaphragms responding to gasoline and air flow.

After years of experience with the 2nd generation Toyota had the option of either adding electronic sensors and actuators to make it easier to control/diagnose the gas tank and evap system, or to go back to a regular gas tank without the bladder, and they chose the latter.

 

The automatic shut-off nozzle was apparently invented in Vancouver, named after the guy, the Seney Valve. My wife was doing home support for him. There maybe were others, collaborators or independents.

 

Compare and contrast, Japan vs. USA gas tank diagrams from Amayama (the year ranges overlap but are not identical):



There are actually two schemas for the USA model that differ slightly. The final schema for both countries seems to be the fuel line from the tank to the front and is virtually identical - the parts listed for that schema are the same but you can see that the routing of the tubes from the canister and fuel pump above the tank are a little different.

Not even the charcoal canister is the same part for both countries. 77740-47040 vs - 47050.

 

The ORVR system for gen 3 is a lot like gen 2, except for the bladderless tank. There still isn't a lot of instrumentation of the ORVR components, so it's still largely up to the human to diagnose issues.

 

Prius Hard to Fill Fuel Tank – Diagnostic Network

I don't have an account there, but it might be that the answer is available in one of the 3 blurred out replies if one does.