It's my understanding from my previous topic post that if I use a 30amp adapter on my 50amp cord it will automatically combine the two lines - right?
yes, but in your case, if you are using female outlets to plug your box into, I would put the jumper in there. Use the 30 amp dogleg for the shore power cord when
50A is not available.
So, you have 3 female outlets, shore, generator, inverter. Shore has both wires passing through. If you need to combine that one (i.e. 30a), you use the dog leg outside.
The inverter and generator have the jumper in the outlet box, so when you plug in your box, both legs are hot.
BTW, how are you charging batteries? If your inverter charges batteries, then it needs power when you are on shore of generator power.