Let water heater cool off
Then stand back.......open T&P via lever and let it BLOW
Then let it snap close Repeat couple of times to blow out crud and scale. Letting it snap close will clean seat also.
Then turn off water. Re-establish air pocket as posted
Then heat water and check for weeping.
No need to replace if you can clean valve and stop weeping.
IF--IF it does still weep.........replace
Both Suburban and Atwood use a 3/4" T&P Relief Valve with short probe (Atwood used a 1/2" threaded one but changed to 3/4" thread in 2004 and newer)
My RV water heater is 10 yrs old and I have opened that T&P 100's of times--still works no weeping