Welcome to Sunline Owners Club and congrats on your new camper.
You are right, you have an issue... If you are looking for long term fix, I would suggest this. This is what I would do. Does not mean you have too, but maybe give you some thoughts on what your up against.
This may sound "extreme" but you are going to deal with this sooner or later. For the immediate moment to go camping until you can get to this, Gorilla tape up the open hole to stop it from getting worse with water or better if you have a garage to put it in out of the weather. The tape will stop more water getting in but will not let any out.
If your not camping for a bit, cover the front corner with a tarp and on dry days, let it air out, again until you can get to it.
Now the long term fix after some thought and more research.
You need to take the corner molding up off the camper. Take the vinyl cover off the aluminum corner channel. Under there are lots of screws. Take the screws out.
The corner has sealing tape sticking it to the camper. A putty knife pushed gently under it can start helping break the suction away from the camper. Using a hair dryer will warm it to help come off as well.
You will have to make a judgment call if you take off the molding to the roof or stop part way. If you are 3/4 of the way, maybe best to take it fully off and reseal.
Up by the roof you will see the corner molding blended into the roof. There is Dicor self leveling roof sealant covering the area of the corner. Apply some heat from a hair dryer or heat gun way away from it and it will soften easy to get under it. Do not use too much heat.
Be careful to not bend up the aluminum corner as you will reuse it.
Now with the corner off you can lift back the bad section and see if the wood behind it is rotted or at least hard and wet. You really need to dry it out totally and kill the wood fungus before sealing it up or it will rot in the wall. If the insulation is wet, it needs to come out.
I have use Rot Dr. Wood preservation, rot repair, and restoration using epoxy resin on boats and homes.
to kill the fungus and seal the wood. Others have use glycol anti freeze.
If you have a rotten corner 2 x 2, then replace it. The camper siding was most likely glued to 1/8" luan board possibly. You will have to confirm once you see it. See if you can get the bad board out of there and replace. Have to research the right glue to glue the side back to the new luan.
Once you get the wood fixed, then find butyl sealing tape, (not putty tape) reline the molding and press it down well and screw it back on.
Now you have created a new good seal that will last for years with no questions.
See this post, it may help. While this is my slide floor, the corners and the butyl sealing tape parts are comparable.
This may help on the roof caulk part
Hope this helps and please post pics of your repair process. Seeing these kinds of repairs are very helpful for others following you. I do not recall someone posting how they fixed their hard sided camper so you could be the 1st!