I think avoiding condensation totally is a real challenge....heating the van means more moisture in the air (and therefore more likelihood of moisture on cold surfaces-windows and bare metal, for example), so ventilation is very important. Raising the roof creates an even larger void to heat, with only a canvas skin (tents anyone?) how breathable are these new canvases? Front windows dropped an inch or so will help, especially if you have wind deflectors to stop rain driving in. Sliding windows are probably a security risk, and will let rain in. I guess there is no easy guaranteed solution.