I had the dealer add one on my Cougar TT and they mounted it near the battery on the inside of the tongue. Make sure the one you use is weather proof if it's going to be exposed to the elements. It seems like it cost me around $100 parts/labor back in 2010. Mine wasn't the locking type.
Chuck, Ann & the Boys (a pair of large labradoodles)
2013 F350 Lariat KR/6.7/CC/LB/FX4
2013 Jayco Premier 321 RLTS w/Atwood Levelegs (now removed)