Hi Marty,
I see BenK link my saga on metal stems... on these pressed out rims. I did get the NAPA metal stems to work. I have since upgraded to 16" LT tires and needed new rims. These rims are made to handle a good stem mount. It may be on the 16" rims there is enough room to emboss a flat verse 15" rims they stamp the hole in the curves of the rim. Heck they could of used 90 degree truck stems in the rim ID. But they cost more $$S.
Hard to see but this is a flat here
I too have heard of folks having the the high pressure brass & rubber snaps in fail. While they are better then the all rubber, they are not fatigue proof enough. I also found my local tires shops never heard of TPM's that screw on the stems. They are use to the internal auto sensors so they have no real good back ground on extremal mounted systems.
If your aluminum rims will not work with an external sensor, maybe look at internal sensors. I had buddy use the Orange system in his 26 foot trailer. They worked for him for many years until he sold the camper.
He had the Orange system. I know Jegs sells it. This may be a newer one then he had.
Orange TPM at JegsOrange TPMConsider this a lead. I have never used them, just recall my buddy using them inside the tire.
Good luck and hope this helps
John