I would separate these bulbs units as far apart as you can. In my opinion you are flirting with a possible fire hazard.
I have seen in my RV ceiling mounted light fixtures using the same bulbs how much burning of the ceiling material that the metal fixtures were attached to. This was only using one or two incandescent bulbs in close proximity to each other. This was a big reason for me going to LED lighting.
The small glass bulbs distance between the bulb and the metal socket is very close which allows the generated heat to overheat the surrounding mounting fixtures..
Being installed in a closed area such as your storage bay area may not be observed all that well and who knows what may happen over time...
I most definitely would be mounting something like this on a metal structure to safely dissipate the resulting heat from the lamps and sockets. Something on the order two-inch high U-shaped metal perhaps...
Just saying respect the amount localized heat you may be experiencing with such a project...
Also be aware 10AMPS of DC CURRENT will drain a battery rather quick... Just think how long two "LEFT ON" automotive headlights which are close to the amount of current drain will last over night.
A HYDRONIC system using your hot water heater and heat registers might be another system to think about. Something along this concept.
Install hot water baseboard heat strips like this
Using propane to keep hot water heated in a close loop does not take much propane. The hot water heater will cycle on and off to maintain heat setting...
I have been thinking about doing something like this to replace my very loud RV FURNACE...
I got this idea from a RV Install this guy did with his smaller floorplan similar to mine.
Good luck with your heating project...
Roy Ken