Do you have a high pressure propane regulator valve, often referred to as the little red regulator valve, near one of your tanks? (Not the two stage change over regulator valve) Those little high pressure regulator valves are notorious for leaking. Inexpensive and easy to replace.
I could smell propane in my driver's side propane tank compartment a number of years ago. Using soapy water and then leak detector fluid, I could not find any bubbles any where. After much searching and reading RV forums, I replaced the high pressure regulator valve and no more odor. When I removed the old one, I could see an oily like substance around the vent hole on the bottom of the regulator. Apparently it was leaking, but the soapy water was not getting to that hole on the bottom and therefore no bubbles.