I'm curious: have you connected your previous and current genny in a way that gives you a true earth ground?

I have an old APC UPS that won't take MSW power either. The internal battery is weak so there's nothing to lose. I thought if I gave it a true earth ground it might work. Otherwise I'll rig it to draw 12VDC directly off an old car battery.

Generator right to the UPS.

I`m not sure if once completed the generator outlets will ground to the house ground or not.