Over the years, there have been LOTS of reports exactly like yours. Apparently, the Apollo Halftime Oven is known for that. The general consensus is to buy a quality microwave and replace it.
We were lucky, the Winnebago we bought in 2007 had one. When we sold the RV in 2017, it was still going strong. Apparently we were the exception.
Google the Apollo Halftime Oven (not just this forum) and read the reviews.
Bobbo and Lin
2017 F-150 XLT 4x4 SuperCab w/Max Tow Package 3.5l EcoBoost V6
2017 Airstream Flying Cloud 23FB