Discounting the LED bulbs, I looked at the incandescent bulbs. It seemed that there were three varieties of bulb with two contacts available. Having no idea what to get and no idea of the power of the bulbs, I simply chose the highest model number. Thus I ended up with 4 Sylvania 7528 bulbs. They were a dollar more than other Sylvania bulbs so I figured they might be brighter.
So, having bought the bulbs, I tried them on the bus. One of the sockets took the bulb but the bulb didn't light. Baffled, I dismantled the light assembly and found the contacts on the assembly had been rotated. No idea how that could have happened but I relocated the orientation notch in the right place and inserted a bulb. It worked well.
I removed all the old bulbs as they just seemed dim. It was hard to see the brake lights in daylight. The new bulbs changed all that. Now the lights can be seen easily in bright sun. In fact, every single light on the bus now works well.
It was suggested that I put an intake fan on the front of the bus. I'm not so sure that'd work in wet weather. I'm thinking more of an underbody air intake. There's an existing hole in the bedroom floor where the bathtub drain used to be. Blowing cool under bus air through that hole would cool the bedroom which is the important area. I'll have to put a couple of filters in - probably a drain cover with mosquito mesh inside and a small vehicle air filter underneath. Again, a CPU fan could be used.