Welcome to the forum, and congrats on your new purchase. I do not think you will have much of a problem getting it level before you open the slideout. I own a 34.5 ft Windjammer with 3 slide outs, 2 of them are deep super slides and while it does affect my level slightly it is negligible. The problem is that you don't want to try to move the trailer while the slide is out so if you slide it out to level it and it is uneven you will have to slide it back in, then out, etc to keep leveling it, not good for the slideout or motor. The stabilizer jacks can offset very minor leveling issues but they are really for stabilizing, not for jacking the frame so I would use caution. Also, it would be a good idea to have the stabilizer jacks down before you deploy the slide to keep some stabilization to the frame. Reverse this when preparing to depart,, retract slide in before cranking up the stabilizer jacks. May be over caution, I admit, but i like to keep structural stress off the frame. Note: If you notice a difference with it being different values slide in vs slide out, make note of this reference point on your leveling device then level your trailer slightly high to this reference point. Then you don't have to deploy your slide to get it level everytime. This is what I do. You'll figure it out soon enough and become proficient.
I discovered on the side my two deep super slides are on that it does drop that side of the trailer slightly so I don't mind if that side is slightly high since it will slightly sag to level when I slide them out. Since you have only an 18 inch vs 36 inch depth slide on one side I doubt seriously you will have any leveling issues. Just Back in, get it level side to side by blocking wheels, chalk wheels in front and back, disconnect from tow vehicle, level front to back, stabilizer jacks down, after that, slide out. You are done minus the obvious hookups.
IMHO i would suggest you install a slideout topper. Some people disagree but consider this; everytime your slide is out the roof is exposed to the sun, rain, leaves, pine needles, twigs, bugs, etc. You need to get all this off before you slide it back in or some of it will slide right into the living area of your camper, including any rain, heavy condensation, etc. That can't be good to sit inside Now you could get on top and wipe it with a broom and towel, but that could be dangerous. Plus no fun if it's raining on departure. I witnessed a guy on a four foot ladder doing this last season and the ladder slipped on the uneven ground and he came down hard and got injured. I felt bad for him cause he was about to drive six hours home with hurting ribs, a bleeding head and arm. I was grateful I paid the extra money to have toppers. Something to consider if not installed. Sorry for the long advice. I truly hope you have a wonderful camping season this year in your new camper.
Carl from AL