The general fan areas are used on matchdays only. In the tour you go to 2 or 3 parts of the stadium, no parts of the tour take in the matchday toilets, surprisingly.
Seems completely normal to me to not check clean toilets for a few days.
Not being funny but have you been to OT? its fecking massive and the vast majority will be used once a week. The megastore, for example, is in a part of the ground not evacuated (east stand). You wouldn't check toilets between the SAF stand and stretford end to run the megastore. The tour doesn't actually go to that corner either.
The museum is in the north stand, but again, matchday areas are not used. Pretty sure mutv wouldn't be using those toilets either!