HELP!!!!!! with last step of inverter install

The inverter chassis should be grounded to the frame.
The inverter ground power terminal s/b connected to the rest of the ground connections and at some point also connected to the frame.
The negative battery power terminal is the only connection direct to the battery. Yes the battery negative is also connected to the frame.
These three should be independent connections.

Some inverters do not like the output ground tied to the battery (through the frame) and must be operated as isolated or portable. Usually the instructions will say not to wire the inverter to an electrical system if this is the case.

The AIMS PWRIX120012S looks like it should wire into your system just fine. Since it has a transfer switch that is how it is designed to operate.

https://www.aimscorp.net/documents/IX1200%20manual.pdf

Inverter owner manuals I have seen say do not use the inverter to run a distribution panel, but despite that, the inverter will run an RV's "whole house" no problem. Apparently, the warning is about distribution systems that are bonded such as in the stick house, but you can do it ok with the un-bonded RV main panel, or run a sub-panel with more than one circuit breaker

Vector inverter manual says--Do not bring the chassis ground around to the neg input of the inverter as a short cut to the frame (via that neg input wire to battery to frame). Like time2roll says, run the chassis ground to the frame directly.

Vector inverter manual says that chassis ground is for reducing RV interference with TVs and radios. I have had no problems with other inverter brands running them "whole house" (converter off) and without the chassis ground. Not saying everyone else would have no problems, no idea.