Microwave Issue

I had an ancient 400 watt microwave for about 6 years and it worked fine with 1k, 1.5k and 2k inverters. It quit a few weeks ago, fan, timer and light working but no heat. I figured it was just time to replace it so I got a new one (500 watt) but it does the exact thing thing, light, turntable, display working but no heat. It works in the house but not on the truck. I'm using a 2k/4k inverter that runs everything else including an 1800 watt George Foreman grill with no problems. I've searched online but can't nail down the problem. Hate to replace a perfectly good inverter but I really miss having a microwave. Anybody of you ever have this problem?