I can only really help you with the levelness. Firstly, do you have/own a level? I use a 2' carpenter's level
Given the feeling that your head is lower than your feet- you want to check if you're level side to side. To do that, you can take the level and put it on the floor inside of the door (you want it perpendicular to the door and going in the same direction sideways to the camper). You want to look at the bubble that is horizontal to the floor and make sure that it's within the lines. My floor isn't entirely level, so I'll check a couple of spots. (Actually, my wife usually does it!)
If you're not level side to side
, raise one end or the other of the level to see which side is lower than the other (the side that you have to lift is the lower side).
(Assuming a park model is the same as a travel trailer for leveling...
If it's not level, you're going to need to get wood underneath of the tires on the low side. That is where things get tricky as you need a truck to do this (not sure if you have once since it sounds like the dealer did delivery + setup). You need boards that are as long and wide as the tires - say a 6-foot long 2x8 or 2x10. If I had to go buy something, I'd get a couple of 2x8 boards and a 1x8 board for "fine tuning".
First, you need to bring all of the slides in. Then make sure that you have something in front of and behind of the wheels to chock them + hold them in place. Then raise your rear stabilizers up. Finally, you're going to need to hitch up to your truck.
You'll pull the camper forward far enough that you can put the wood down where the tires were previously. Put them in place and then back the camper up onto them. Go check level and add/remove as needed until you're level side to side. It'll likely take a couple of tries. Like I said before, my wife has it down to an art- she can look at the lift of the level and say, "we need X blocks" where I'm like "duh.. I'll try this many."
Once you get leveled side to side, you can use the tongue jack to level front to back by simply raising/lowering the front of the camper until you're level the other way (you turn the level so that it's parallel with the door/camper on the floor).
Once you're level front to back, you re-chock the wheels, unhitch from the truck and lower all of the stabilizers.
If it was already level side-to-side and front-to-back
, your bed could still be out of level. I know a few people have reported that. They've somehow made ramps to lift the head of the mattress up a little to alleviate the feeling.
Though, I will say- this might be a time that you try and talk to someone in person who has experience with this stuff. A trailer rolling away or dropping off of the hitch can be dangerous!