The BMS (battery management system) in your Li batteries SHOULD have protected them from mis wiring. YOur old batts were 4x6v ea which means they were wired in series/ parallel. Your new lithium, if each is 6v will be wired the same, but I suspect you have (4) x 12
v li batteries? is that right? 12v batteries are wired in parallel (+ to + and - to-). You ask about two negative cables, my coach (4x 6v) has two negative cables, this is done to minimize voltage drop to the inverter. If you trace your cables, the two negatives will join at a big battery terminal near the inverter. As far as connecting your li in parallel, check with who you bought them from, as they might limit the number of batteries connected that way (parallel). If none of this makes any sense, find an RV electrician who knows his stuff. And, you are right to go to lithium, but with that big of an investment, you better get some real help to install them correctly.