That's a tough space. You can do something in there, but it may not be much at all. How are you going to ventilate the space? Can you put a 6" vent hole in one of the 1.5' wide walls or out the top? If you can't do that it's going to be very difficult to fit any hid in there. When you want to save every last inch in the room for plants like this, you will want to consider placing the fan outside the room pushing into the filter, instead of having them both in the room pulling air out. Just make sure you use a dust pre-filter before the carbon filter.
The other big problem is that 4'10" x 1'6" is not at all a footprint that an hid is going to cover well. Two could, but you are not fitting two hid's in there unless it's 2x250w and it sounds like that is out of your budget.
One option is to put a 400w bare bulb right in the middle vertically and just grow one nice size plant on each side of it. That's probably your best bet, if you can cool it. Put a 6" vent hole right above the bulb and the fan above it, outside the space, pushing through the filter and it'll work as long as it's not like 80°+ in the camper. Veg in there for a month then flip and get 8-14 ounces every 3 months easy.
The other option I would recommend are 55w PL-L fluorescent lights. A website called AH Supply sells nice kits with reflectors or now you can find 2x55w and 4x55w fixtures for cheaper on hydro sites.
HID's kill flouro's all day long but cooling 400w in that space is going to be rough. You need a decently cool ambient temperature/intake air for that to work. In a non-air conditioned winny, not happening in the summer.