Looking at the floorplan for a 3870FB, I'd start by removing some of the bolts holding the coroplast under the bedroom slide, pulling it down enough to peek inside and taking a look. My guess is the tank serves both toilets and might be located between them. Alternatively, taking a look on the roof for tank vents might lead you to where it's located since the vent pipes tend to be straight into the tank.
You might also find it by removing the rear basement walls. The tank may be located (or visible) nearby.