Okay, Lots of good advice here, however, I have a slightly different view on some of it.
First let me say that we have our own Travel Trailer. The claim is that it sleeps 8, however, the reality is very much different. It could sleep 8 as long as at least 4 of the people are TINY.
The floor plan of our TT is below:
Now you can easily get 2 people on the queen bed and 2 on the bunk beds (1 on top limited to 150 pounds). However the sofa pulls out and folds down (like a futon) but I have had to sleep on it and I am here to tell you that there is no way Me and another would sleep on it. No room. Same with the dinette, it will collapse down to a bed but putting 2 larger people (teens/adults) on it will be very cramped and you will be cranky.
Last time we went to the fort (Thanksgiving time of 2014) it was Me, DW, DD9, DD17 and her BF20. DW and I were on the queen bed, DD9 was top bunk, DD17 was bottom bunk and BF20 was on folded sofa. We stayed at the fort for 8 days. It was cramped, there was not enough (easy) storage for clothes (BF20 kept his clothes in "Bins" stacked next to the sofa) and then there is the "Conversion" time (cleaning up the sofa from bedding and making it back to a sofa from a bed). Still, we were okay with it. It was more just for sleep time, an occasional meal, etc... If we wanted stretch out room, there was "Outside".
We are not "Rope Drop to End of day" folks. We might do Rope Drop or we might stay until end of day but rarely do both in the same day. We usually have a Down day at the fort (Pool, movies, etc...)
We also would frequently use the comfort stations. First, it's just easier and more spacious then our tiny Bathroom. (not enough room to change comfortably in private). Second, at least for me (the DH) and the BF20, waiting for the Ladies to do whatever it is they do (that takes them Forever) is just a waste of time for us. (in fairness typically only the DW and DD9 would shower in the TT). Third, with frequent use, the tanks (grey/black) fill up quicker and require frequent draining. (it's not advised to keep the drain pipes open because sewer gas can come back up)
So while we were okay with all of this, I mention it to illustrate what you could be faced with. With 8 people, even some of the nicest and largest RV's/TT/5ER will leave you "Converting" tables or couches to beds plus the storage space for clothes will be tight. Of course this depends on your kids and how flexible they are, how big they are, etc......
The downer may come if the weather is bad. You kind of get stuck with "Indoor" stuff. (TV in the RV or going to a mall or similar) and that's when tempers can flair if you are cramped inside a camper (although some of the fifth wheels are HUGE).
We did have some "Temper" issues which you will get anytime you are in closer quarters. I have never stayed at a deluxe so I don't know the difference in size/space. So some of the larger RVs might have enough space to really compare (especially since you get the Outdoor space)
Don't want to discourage you as this could be an awesome time. Only you know how your family is (their likes/dislikes/quirks, etc..) I think it would be an awesome experience for you and your kiddos and you would certainly miss out if you didn't even give it a shot. You will find that most of the RVs have many amenities that you would find in some of the nicest hotels (and even more, full fridge, Stove, Oven, Microwave, TV's, some even have (gas) fireplaces).
I might suggest trying to find a way to do a Short (weekend) trip to a local campground as a "test". I realize it's not always possible, but if you can do it, I think it would be worthwhile. Certainly figuring out some of this things that you might not have otherwise (Like Pahunter pointed out in his "Other Considerations")
Either way, good luck. I am sure you will have an awesome time at WDW either way.