That is a TEMPERATURE and pressure relief valve. It will relieve when the temperature reaches 210 degrees and when the pressure reaches 150psi. An RV plumbing system is so small that it doesn't take much for the expansion from heating water to reach 150psi which will cause the T&P valve to relieve. It could be that your water is too hot, but not likely. If this valve leaks continually, it may be worn out or the seat and washer may have hard water deposit buildup. Or, you may have a temperature or pressure problem. Replace the valve and see if it stops dripping. If it doesn't, work out why the pressure is so high or why the water is so hot.
Bruce