I went to NAPA and bought a certified one, never use it directly to check the pressure, just to check the accuracy of the others I own.
One of them has an aluminum housing and hoses to set two tires at the same time, supposed to be accurate to 1 psi. I also have a digital one that's supposed to be accurate to ˝ psi.
Yes, I'm a fanatic about tire pressures for the motorhome! A good TPMS is also worth the money. Had a TST system in the Dutch Star, the Magna has a Smartire II but they're out of business. Luckily ours is still working, not sure for how much longer though. When it goes I'll just buy some more sensors and put the TST on.
2009 45' Magna 630 w/Cummins ISX 650 HP/1950 Lbs Ft
Charter Good Sam Lifetime Member, FMCA, SKP
RV'ing since 1957, NRA Benefactor Life, towing '05 Odyssey