If you don't have one of these already, pick one up:
https://www.amazon.com/Bastex-Tester...xpY2s9dHJ1ZQ==
This one is at Amazon but you can get one at Lowe's or HomeDepot or even Walmart I suppose.
Plug it into the inverter and if it checks out ok plug it into the T/S.
I had trouble with ours occasionally with the GFI tripping. I leave one of these testers plugged in all the time so when We're travelling I can just peak in and see if the inverter is working OK.
If you tested hot between black and green but not on black and white, you have something wrong. This tester will tell you what that is.