I think getting a window installed would be much easier than getting a door replacement. A door replacement would have potential issues with alignment, and paint matching. Adding a window is a task that many DIY types have accomplished, so it's probably the easiest/cheapest route. Also you could install an RV style window instead of a standard van window which is almost certainly a plus for van dwellers. I don't have a window in my slider and I don't think I would want one. I suggest trying to live without the window first, you might find it works. Benefits of no window are easier for stealth, insulation, less weight, and you don't have to cover the window for insulation purposes. Most people living in vans cover their windows if they have them, but that is just my opinion based on what I have seen.
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