Propane is a heavy gas and will seek the lowest point in the coach. You should place your detector on or near the floor. The 'smell' you smell is an additive and not part of the gas' natural state.
I've only seen the soapy water test improvised once where it worked. The spray bottle is good and can also be awkward or ineffective in tight spaces. If you can reach it with your hands or finger tips, the following may be an alternative solution.
What you'll need:
1) one small container
2) paper towel
3) dish soap
4) flashlight (for dark places
What to try:
1) Place a generous amount of soap in the bottom of the container.
2) Add water, a little at a time, and stir thoroughly with your finger until soap is dissolved (you can't spend too much time on this step).
3) moisten (not drown) the paper towel under a faucet
4) dip the paper towel in the container and apply to the joint
5) wait and watch for bubbles
The lines in our RVs are not high pressure. You may have to exercise some patience and wait for the 'real' bubbles to appear on the joint, particularly the tiny ones (keep low pressure in mind). It may take a while.
The advice offered to have a professional address the problem is not a bad one at all. This is a different approach and comes with no warranty or guarantee.
Sorry, had to include that for legal reasons. It does work. Ask me how I know.
Trouble free since 2017.