How long is the warranty? Several SGP advertise 3 year warranty. Think you have some left if 3 year?
I think ppl are over loading inverters and killing them. Manufacturers should probably reduce the advertised loads but think it is a marketing thing so not going to happen. Electric air compressors use to claim 5hp 120volt 20 amp breaker. The only way they were broke from false advertising was a law suit.

Most gas generators are really only rated at 1/2 output. The instructions often tell ppl that. Most get raggy on upper end - dirty power and dependability drops if run at higher rating plus they get hotter and fuel consumption goes way up. Ppl buy undersize generators and have same problems. Then they cuss the generator when it is the advertising marketing ppl at fault.
I think we are maybe seeing some of that with these inverters and maybe for longevity the loads should be significantly reduced. Despite the advertising. I would have had them in parallel if it were me if you wanted more. We all want more. The problem then becomes we will add more load.
I would contact SGP and inquire about warranty - look at your paper work.