There are many companies that make RV/marine windows, single/dual pane, fixed/slider in about any configuration you need. ie, dont know anything about them but just one,
http://www.rvwindows.com/products/rv-elite-dual-pane-windows-series-1800/.
Likly also be ordered thru most RV mfgs, in PJs case as rebuilding camper, replacement shoudnt be hard to find-they are not cheap.
Which some have retro'd their own quasi dual pane rear door window.
KKELLER14K for one, its in the TCU thread/exterior section. If I had a problem with our door windows I'd try that, but we dont. I assume the simple door seal/door construction leaks enough air or cold enough to keep fairly dry as its the window that has least condensation.
On the round to it is our cabover window, its the worst 'water generator'. There is a flat flange that I will seal a piece of plexi to, think it'd help. I had the opportunity couple of winters ago, because there is an actual secondary flange that would accept a second window, I always thought if/when removed id make semi dual. However it was when we used to get rain so I didnt spend the time needed. Ive not seen any long term results if these attempts help.
If though I was to spend couple of hundred (not) Id opt for the Edwin opening window, velcro some foil bubble ins. for winter use.