If you have a single gray and a single black tank, each 40 gallons, that's not a lot to work with. The way it is, with galley and bathroom sink and toilet draining to the black tank, you'll fill the black tank pretty fast. But if the galley, bathroom sink and shower all drained into the gray, you'd fill that tank pretty quickly. Depending on how you use the water, a change might not be for the better.
Maybe call Heartland Customer Service at 877-262-8032 / 574-262-8030. Have your VIN # ready. Ask them if the way yours drains matches the engineering drawings. If it doesn't, they'll probably authorize your dealer correcting it to match the drawings. But it may not help you very much, if at all.
You might take a look at the plumbing drain pipes. Bathroom shower and sink are usually pretty close together. I might not be hard to re-route the bathroom sink.