Hi Maggie,
Grounding/Safety, is definitely something you want to be clear about, especially on the AC side. Part of the reason you probably have not found a clear answer is because many people don't feel comfortable giving advice on specific safety questions since its a serious issue and very context specific. And because its a sometimes quite complicated topic, especially in the context of a vehicle. I am not qualified or comfortable giving this sort of advice.
I will give some general thoughts though.
1. To make sure we are on the same page can you explain exactly what you mean when you say "wire to use for my
AC ground to chassis"
2. The more info you provide the better help you will get. At a minimum your inverter model (and ideally link to the manual), and a brief sketch or outline of your system.
3. There are some good resources on grounding in the
resources section. Filterguy and I collaborated on (most of the work and all of the expertise was his) a few beginner resources on grounding that are a little daunting but helpful. I uploaded the grounding section of the National Electrical Code as well as the ABYC (marine code) which is a good resource for the vehicle crowd as well.
4. I would think in general, and I'm not an electrician or electrical engineer, that you would be safe using a chassis ground wire equal to or greater than the current carrying conductors in the same circuit, and greater than the rating of your fuse for that circuit. But this is something you would want to double check. In some contexts you can use a much smaller ground wire for certain purposes, but for the cost of a little extra copper, I'd just as soon keep it simple and err on the side of caution.
5. Grounding and safety is a subject you want to be clear on before you start installing things. If you aren't confident/comfortable with your understanding, it may be worth paying someone qualified to at least double check your design, or help you design or install.
I will ping
@FilterGuy, he is a really helpful guy, and has more expertise and experience helping people with system design, and gives good advice/feedback