Bath tub, hot tub or small kiddie pool. Suspend a tarp from trees. Carry it to your lake and submerge it. You don't even need something that deep. Just roll the tank over. I have never been let down with submerging in water. Regular soap IMHO is to thin and a leak will blow away the soap without it foaming. I use a product called Big Blue when I soap test.
50 pound drop in 24 hours is small enough that you cannot hear it.
Compression fittings are bad. If they are not leaking now they will leak.
The gauge and fittings you are using to pressurize the system could also be the source.
Any Home Depot or Lowe's fittings should be discarded. Most of them are garbage.