Secondary Generator Wiring
The trouble with back-feeding portable generator power through any of those welding or RV plugs while the transfer switch is set to generator position is the same problem with relying on turning off the main breaker before doing the same: it's manual, not interlocked, and sooner or later somebody is going to get it wrong.
Instead, daisy-chain another transfer switch. It could be another RTS series switch if you're so inclined, but the transfer switch could also be as simple and inexpensive as a main-breaker type panel with a generator interlock kit for a branch breaker.
I gather you'll want to keep the automatic switch-over for the 22 kW generator working and add a manual alternative in case the 22 kW breaks down or you just decide not to use it. To do that, add a 150A or 200A main-breaker panel. Wire the main breaker to the 22 kW generator. Add a branch breaker in the 100A-150A range and wire that to the &quot;generator input&quot; connection of the existing transfer switch. Finally, add a 70-100A branch breaker in the designated &quot;generator backfeed&quot; location of the panel and wire this to an inlet socket.
With the above arrangement there are two operating modes. Either the main breaker is on, allowing 22 kW generator to connect to RTS input, or else the generator interlock is activated so that the inlet for the portable generator is connected to the RTS input.