Convection side of Microwave takes forever to...

Question

I have just acquired an RV with a Sharp sensor Microwave/Convection oven. It appears in excellent condition and the Microwave part works great. When I use the Convection part, I can see the element heat up on the left side and hear a fan running but after 40 min. at 325 degrees, my small casserole barely got warm on the edges. The model is an R-1870F made in 2005 if that helps any. I would really appreciate any help you can give me. Betty