You don't need 15" floor width at the propane tank. You only need a little bit of floor to set the flange on, and to hold it higher up with a strap. This would let you put it much lower. The tanks are a very standard size so you can have confidence using one to make patterns. Look at tanks on the front of camping trailers for ideas. You could then build a v-shaped compartment around that, which would open up your design space.