When the trailer sinks into the ground or a tire goes flat, those "stabilizers" will destroy the slide as well as damage the surrounding walls.
There is no way of knowing exactly how much weight a slide will hold. You can go outside and look to see if there is flexing when some heavy people step into the slide. It is a concern and you should make an effort to minimize the weight, but the difference between the two types of furniture shouldn't be an issue unless you select something that is extreme. Keep in mind that whatever you buy must fit thru the door.
A fireplace probably weighs less than the box it came in. If you add a fireplace, I would add a dedicated outlet for safety reasons. The self-contained outlets used in RVs might cause problems with a heavy load.
If KZ says the dinette is for 4 people you could easily calculate 100lbs each being conservative so 400lbs. Most of use are way above 100lbs so figuring 150lbs your at 600lbs, 200lbs would give 800lbs. You get the idea, couch weight that much??
Like lynnmor said the fireplace is minimum weight and a dedicated circuit would be great. KZ has mine on the same circuit as the fridge, TV, and satellite, every so often I have to use the breaker to reset the fireplace heat sensor when it will not shut off. When that happens the satellite has to be restarted and the TV tuned back on.