My camper (not loaded) weighs 265. I think I add another 50-60 to that including an extra tarp with poles. I keep the tongue weight about 25-27 by adjusting the distribution of the pack. Use a fish scale from Walmart to check this. I found that works best for me performance-wise when I pulled a bigger camper with my Gold Wing and still works well with pulling the Mini-Mate (smaller, lighter camper) with the ST1100. As to max trailer weight that a ST can or should pull? That is a debate that keys into the rider experience, skill level, and what level of risk you will tolerate. I've heard many say don't pull more than 25% of vehicle weight but that would exclude many trailers, cargo and otherwise, that mc riders pull. If your trailer is 150 and you pack 75 in gear (not uncommon for cargo trailers that hold more than my camper) you're already past 1/3 of the vehicle weight and approaching 1/2 vehicle weight. Looking at bigger rigs, trucks pulling RV trailers, the trailer is the same or much more than vehicle weight. The real detail to watch out for as to ST1300 capability for weight is how your hitch is mounted on the bike and how much that part of the frame can handle. Load weight of your ST includes rider, passenger, cargo in top case and panniers, as well as tongue weight. Other than that it is up to your skill level, that will build with each time you pull, keeping in mind you have more *mass* back there that will affect how fast you can start up and accelerate, and how quick you can brake and bring it to a stop. Expect and allow a LOT more distance and time for both.
I always figure that my trip from home TO the campsite is to transport the trailer/camper and leave it there. Then I can go have fun riding without it while there. So I don't bemoan the extra load on the back end during my trip to the site or back home. One of the previous threads in this forum has a video from Dale of Backroad Outfitters showing him pulling the same camper as mine, from the campers' perspective, and not really cutting back on his riding style all that much. Just always be aware that there is an extra load being pulled behind and all is good.