Don is right - Smartgauge method #3 requires 2 busses separate from battery bank.
Use method #2, it's good enough.
You can put up to 4(?) cable lugs on one Pos battery terminal. So you have 1 from MPPT, 1 from inverter, 1 from trailer load, 3 total. Plus, much thinner wires from MPPT volt sense and temp sensor if you have it. I don't see a problem.
If you need more than 3 cables on one terminal, you can install a terminal extension (or whatever it's called) - a lug that is bolted to the terminal and lets you put more wires on it.
To avoid crowding, use as short interconnect cables as needed.
This shop will make you custom length of any cable, with quality lugs and shrink tubes installed. Very low labor surcharge - I think I paid $2 per cable, on top of cable and parts cost. Their parts are local but inexpensive, cheaper and better than Amazon cr-ap.