[JRTJH]

There is an easy way to re-establish the air gap. First turn off water to the trailer and turn off the water heater. Then turn on a faucet inside. It will only drip for a moment and pressure will be relieved. Leave that faucet open and go outside, open the pressure relief valve lever. When it stops dripping, let the lever snap shut. Go inside and turn off the faucet, turn on the water to the trailer and then turn on the water heater. Open the faucet and when it stops "sputtering" you should have re-established the air gap.

Alternatively, you can remove the anode rod/remove the plastic cap and drain the water heater, however I'd try the above method first.

It should stop the dripping. If it doesn't then you'll need to remove/replace the pressure relief valve. Take it out, and take it to any hardware store. They can "match it up" with a replacement. If you order one from an RV parts store, expect to pay about double the hardware store price.