Not sure how anybody on the internet, would know where the prior owner installed an inverter......
But if he did, he would have had to tie into the 12vDC house battery or buss somewhere. So I'd start there, and try to understand what all is connected to the buss.
Is there a control panel somewhere that you don't know what it does?
If you find the inverter, get the brand and model numbers off of it.
Since you just bought the coach, it might be easier to call the guy you bought it from, and ask where he installed it, and how he got it to work.