Tom that is a spectacular machine but YIKES indeed!
DryHeatDan, I'm certainly no pro at this either, I just know enough to get myself into trouble. The item Tony suggested is a fantastic item for anyone who regularly tows to have in their toolbox and it is quite inexpensive. They are commonly available anywhere you find trailer towing accessories. You plug that into the connector and the lights will tell you what things (signal, tail, brake, etc) are working from the tow vehicle side as you activate them.
If your only problem is the stop/tail feature on the right side and everything else works properly that tends to suggest the problem is in the trailer. It IS possible that the bulb in that position just happened to go bad, even though it may have been working fine the week before you had the connector installed on the vehicle. There might also be debris in the bulb socket or a broken wire in that circuit anywhere between the bulb and the connector.
You can also use something like this...
https://www.amazon.com/dp/B07F2GWNRV/ref=sspa_dk_detail_1?psc=1&pd_rd_i=B07F2GWNRV&pd_rd_w=QoS5G&pf_rd_p=4269e1a0-a218-4fbd-9748-1cd337d2f2a5&pd_rd_wg=8L0Lv&pf_rd_r=QGCK5R2VX40HNN2C9ZFZ&pd_rd_r=2006a46c-a977-4d2e-bf14-699ae8526fc9&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUEzNE5HNUVTN1lTOEtSJmVuY3J5cHRlZElkPUEwMjM2MjYwMVVHUEg3ODU4S08wQiZlbmNyeXB0ZWRBZElkPUEwMjEwOTg1MU9BVThPSE1SV1YxQyZ3aWRnZXROYW1lPXNwX2RldGFpbCZhY3Rpb249Y2xpY2tSZWRpcmVjdCZkb05vdExvZ0NsaWNrPXRydWU=...to test the circuits in the trailer. There are a few different varieties of these. You can use one that contains a small battery to test for continuity or one without to test the condition of a circuit using the vehicle's power.
All things considered, I would first try replacing that one bulb...
