I just had same type problem with a new tire on my boat. I got 5 new tires and valve stems. Used the boat several times, pulled about 100 miles total. Each trip check psi at or near 50. Last trip I really angled the trailer hard into driveway and 2 days later, 1 tire flat. I removed it, rolled it around looking for debris and found nothing. Aired it up, Stem checked with soapy water and was OK. Left wheel/tire at dealer for 4 days and nothing found wrong. 3 weeks later and not moved it is still at 50 psi. I and tire shop have no answers. Did the hard angling into driveway have anything to do with it? maybe.
2013 24RKSWE (27ft TT) Cougar 1/2 ton series SOLD 10-2021
2013 Ford F350 4x4 CC 6.7 engine, 8 ft bed, 3.55 rear end, lariat package
Retired from Oregon State Police in 2011 than worked another 9.5 years as a small town traffic cop:
As of 05-2020, I am all done with 39 years total police work. No more uniforms for me.