Our GWV Legend has an Espar D5 hydronic heat/hot-water system. I added an extra glycol loop that runs along each of the exposed water and sewer lines, bundled in insulation. In the winter time, I open a valve that diverts the hot glycol (which circulates continuously) into this loop. I also insulated the fresh and gray tanks (which have 12VDC heating pads). Finally, I wrapped a few remaining pipes and valves in 12VDC heat strips. So, I essentially have a 4-season van without the space penalty of inside plumbing and tanks. Advanced RV uses a similar setup.
Formerly: 2005 Airstream Interstate (Sprinter 2500 T1N)
Now!: 2014 Great West Vans Legend SE (Sprinter 3500 NCV3 I4)