Yes, extending the slides will cause more weight on one side of the trailer and will cause it to lean, even though it was level before extending the slides.
What you need to do is set up the camper and extend all the slides, then take a measurement how much out of level the entire camper is (with the slide out) .... and no jacks down.
If it's off on one side by (say) 1.5 inches, then you know in order for the camper to be level after the slides are out, you need to raise the opposite side 1.5 inches BEFORE extending the slides. This mean, put blocks under the tires which will raise the leaning side UP 1.5 inches (before extending the slides).
Now.... if you attach a bubble level, attach it at this point, with one side raised the 1.5 inches. Yes the trailer is unlevel. BUT!!! When you extend the slides, the weight of the slides will now cause that 1.5 inch raised side to squat, and it will NOW be level when you walk in it. (Now, you put your stablizer jacks down). Your bubble level will be way off. But the camper floor, refrigerator, doors, everything will be level.
In time, you'll just be able to eye-ball the right-left level and probably wont' even need to use the bubble level any more. BUT... that's how you do it.
Yes, it is very common with trailers with only 1 slide or 2 slides on one side only to squat on the heavy slide.
Some folks put extra leaf springs on the heavy side of the camper to compensate for the lean. But if you want to go the cheap route, just put wood boards under the heavy side that squats.