My Multiplus 2 is not charging on leg 2…. what could be the issue?
So what’s the purpose of L2 then?
You didn’t mention which MultiPlus-II you have. Are you in America and using one of the 2x-120V models? If so, L2 is simply pass-through when using a 50A shore power connection. This is explained at the beginning of the manual, section 2.1.
Review the block diagram above.
Is there a Victron Inverter/Charger that will utilize the full power of my Onan 8k QD generator, or shore power for that matter, and take energy from both legs so I’m not wasting fuel or time?
Not as a single unit.
You could use two single phase MPs and set them up as a 180° splitphase system, so you get charging and true splitphase power from both grid/generator and inverter.
Or you could add a pure charger on L2. Lets you charge on both L1 and L2, but only gives you 120V while on inverter (essentially what you already have plus some more charging power).
In any case, make sure the batteries are actually able to take additional charging power.
If your genset is behind a transfer switch AND your genset has in-phase outputs, you can tie L1 & L2 together.
My 24V multiplus charger maxes out at 70A DC before it maxes out the genset on L1.
Does your generator output two legs of 120V that are in phase with each other or does your generator output 120V/240V? Look at the sticker or plate on your Onan that shows the model number. It will tell you if it outputs 120V only or 240V. If this all doesn’t make sense, then I would suggest you call your closest Victron distributor and ask them to guide you in the right direction. You can find them here: https://www.victronenergy.com/where-to-buy
Simply put, if your generator outputs 240V then you can get a second inverter and run it in either split-phase or parallel. If the generator only outputs 120V, then get a second inverter and run them in parallel. I still recommend working an experienced Victron distributor. Even with two 3kVA inverters, the absolutely max charging you can do is about 4800W. If you want to load up the generator even more, then you need larger Multi’s, additional multis, or add extra battery chargers.
and yes I’m an American trying to figure out stuff above my head! 
Would buying a Victron 5000 inverter/charger do the trick?? Again, I have an Onan/Cummins 8000 QD that has so much more power to give than is being utilized by this inverter… My thinking has been to build up my battery bank and use the generator to recharge when out boon docking so running it and only getting 1900 or so watts is, well, crap.
There’s no shame in trying to learn things, but I think you would be better served working with a local distributor. We are based in Ohio so if you’re reasonably close we can help you.
To answer your question, a 5000VA inverter will have a 4kW maximum charger. It’s still not going to load up your generator fully. It would leave a few kW of generator capacity to run AC loads while charging the battery.
ok, can we chat on the phone to discuss when you are available, please?
Michael 540-686-0432