I don't own a Thor, so take anything I say with a grain of salt. Also, I only have information for 2006 through 2009 Ford F53 chassis. Yours may be different- although Ford chassis wiring doesn't change much from year to year.
So, based on a 2006-9 Monaco, built on a Ford F53 chassis:
Look for two connectors inside the left-hand (driver side) frame rail, near the rear of the coach. If you open the PDF file for drawing 16621903 on this post
, the connectors and wire color/pin info are on the left.
Assuming your coach is wired the same, for the "trailer" marker lights you'll use the brown/white lead on the 4-pin connector.
I believe it's possible to purchase the Ford pigtail that plugs directly into the two connectors, so you don't have to splice into them. If memory serves, the cost was under $100. It's also possible that Thor used the pigtail for the 4-pin receptacle, and the two connectors are already there, with unused wires bundled up.
If you are looking for a fused 12V connection for something like a battery charge line, you won't find it on the two connectors that Ford provides. On the F53 the "body builder" (in your case, Thor) provides the circuit and wire to that pin on a 7-pin receptacle.
You could ask Thor for a copy of their diagram for your coach as it might come from the factory with a 7-pin receptacle. It would come from the horse's mouth, then, instead of mine (no comments about what other kind of four-footed pack animal I resemble, please). ;-)