Low point drains (one for cold water plumbing system and one for the hot side). They are the lowest point in the plumbing system and intended to allow emptying all plumbing lines to HELP in winterizing your RV. They are located at different places on different models of trailer and some are not capped but operate from valves located above the floor.
2011 F-150 HD Ecoboost 3.5 V6. 2550 payload, 17,100 GCVWR -
2004 F-150 HD (Traded after 80,000 towing miles)
2007 Rockwood 8314SS 34' travel trailer
US Govt survey shows three out of four people make up 75% of the total population