I have done a couple hotel mystery shops for spotcheckservices.com
I have done one for SQM. That one is fairly easy and reimburses only 50% of the hotel room. But I have seen other ones on their website that reimburse the whole amount. I haven't seen those lately.
performalogics has an occasional motel mystery shop, I haven't done them.
I do not know of a company that specializes in mystery shopping hotels.
Businesses move from one mystery shopping company to another so the ones I've mentioned may not be up to date.
Good luck.