I've thought about making a sealed propane compartment within the my trailer (sealed from the inside and vented to the outside). ALL truck campers use this method to carry propane. They use a composite plastic box sealed to the camper wall. The door to the outside is vented. I'm not sure that installation would save any money though. I've never seen an externally vented storage box for sale. One thing you could do it get one off a truck camper that is being salvaged. The doner camper I'm using has a bottle box but the propane bottles are in the horizontal position. So for now I carry my two 5 gallon bottles on my tongue. The mount cost $25 at my local RV store.
http://www.etrailer.com/Trailer-Cargo-C ... /1801.htmlA plastic cover cost another $25:
http://www.campingworld.com/shopping/it ... hite/49349 .
As for the bike rack, I don't think you'd save any money with your method. You could buy or fabricate a strong tire carrier that's mounted to your tongue. But it would have to be over built to accept a bike rack. And I think those racks need a bigger tire to work. Plus they seem kind of cheesy. You'd be better off buying a separate bike rack. How about welding a small section of 2x2 metal tube to the tongue and then use a receiver hitch style bike rack?
http://www.yakima.com/shop/bike/hitch. Personally, my bikes ride inside my trailer

I do like your idea of mounting a tongue box to carry the bottles and a tray mounted bike rack on top of that. It would be the most professional looking option. Maybe also the most expensive. To play the devils advocate though; be careful of your turning radius. If the bikes stick out too far they could hit your tow vehicle.