So, good information from other posters, yet they appear to be assuming you're understanding how the problem is created. If you understand things like air moisture content, thermal bridging and such, cool.
If not, I'd recommend you watch a couple of videos on condensation and read about thermal bridging.
Per the idea of screwing a piece of wood (cross member) between the ribs of the ceiling, then screw the firing strips into those into the cross member, you still have a screw going into the metal and into wood...it's just that it's going into the cross member versus the firing strip. So, you can still have condensation form.
To address this, you might consider using a really good construction adhesive to glue the wood cross members in place, then screw the firing strips into the cross members. This way, you have no screws into the metal of the bus.
I screwed my firing strips directly into the ribs and coated the screw head with silicone caulk. This insulates the screw head from the air.
Of course, you have to insulate all the metal from the inside air, so whatever medium you choose (spray foam, rigid foam, wool, etc.) is key.
Once the insulation is in, it's time to combat the real culprit of condensation....the amount of moisture in the air.
If you're using any propane appliances, make sure they are vented to the outside of the bus.
When taking a shower, isolate the steam as much as possible and exhaust it to the outside of the bus.
When cooking, exhaust the moisture to the outside of the bus.
Keep the air moving! It's harder for condensation to form when air is flowing.
Absorb the moisture out of the air....dehumidifier or a wood stove are probably your best bet.
Bottom line, keep the humidity low. The colder it is outside, the warmer it is inside and the more moisture in the inside air, the more condensation you'll have.
Hope this helps.