When you say you are replacing the "inverter", I am assuming you are talking about the AC to DC "Converter" installed on most RVs and travel trailers (an "Inverter" changes DC to AC and usually just plugs into a DC "cigarette" outlet).

Here is what I have learned, often times the original manufacturer is still in business and they have a replacement Converter that will fit into the original hole. Some even have upgrade kits which take advantage of the original housing (this can save you many frustrations of modifying your existing hole to fit a new/larger box into).

If you have already purchased a new converter, seriously consider returning it for an upgrade kit from the original MFG, or get one that is physically smaller than the existing converter because squeezing a larger converter into a small hole would probably be challenging and will lead to foul language.

By-the-way, you want to get a converter with a "Smart" charge capability. "Smart" charge converters usually have four different charging modes for your battery; boost, charging and trickle, additionally, the "Smart" chargers have a "De-sulfation" mode to help extend your battery life.

Here is a real good Video explaining the installation process and video(s) of the actual install (this is the intro to a 5 part installation).

https://www.youtube.com/watch?v=pjbtUhXzcxc

Additionally, ensure that both ends of the grounding wire are clean and are making good electrical contact (speaking from experience).

Good Luck !!

As always,

Happy Scamping !!!