Check or Google for original documents regarding your mh.
It sounds like a po had modified something or something has failed.
If the generator/shore switch is factory it should look as such and have a label of sort.
If just there then likely po added it.
Get it fixed now as someone can be killed by this.
First given older class c it likely did not have an ats so look at generator and in other places for an outlet that the shore cord can plug into.
If you can find one then the factory setup was plug cord into genny or shore, no switch.
If no plug or place where a plug was then you need to get or make a mts.
This is a simple two polarity two position switch rated for your load.
Check the output of your genny in amps and get the next larger rated switch.
If 20 amp genny get a switch rated for 25 or 30 amps at 120 or 240 volts ac.
Voltage matters in switch ratings.
Many here will provide additional help and if not sure post photos and model or part numbers.
Tony & Lori
1989 Country Coach Savannah SE