Also the Singapore Airforce do indeed own several KC-135R tankers based from Changi WSSS Airport.
I highly doubt an Aircraft the size of a KC-135 would simply be stooging around DRW at about 1000' for an hour over the suburbs........
Maybe the Aircraft was conducting circuit training? Certainly a possibility, it would explain why you saw it at about 1,500' gear up ( downwind leg ) multiple times in an hour.
