Need advice

We have a pole barn at our home where we park our 5er with 30amp service. I had a 30amp receptacle installed also to keep the rig plugged in during camping season so the fridge stays on and the smart convertor keeps everything topped off. Well, the in-laws purchased a 5er last month and they will be keeping it under our barn also. It too has 30amp service. Here is my issue. We only have one 30amp receptacle but they want to run there fridge and convertor also when stored under the barn. FIL has a 50ft heavy duty 12/3 yellow jacket extension cord with the large female 3 way end that both rigs can plug into with the adapters. The only thing that will be on will be the 2 refers and convertors. They may be a light or two on shortly when returning from a trip to clean and unload but that's it. He says it will be fine, but me being safety minded and not a fan of fire, I'm skeptical. Any input or advice would be appreciated cause I don't want to tell them no.
Thanks,
Scott