Mommy R,
We were in a similar boat last year. We decided to "come clean" and simply tell my Mom, we're not up for it. That it was a personal family decision to opt out. I felt it was my responsibility to have this conversation since it was my mom. (I think you should encourage your husband to do the same.)
IF you feel like continuing to go on vacation with your in laws, then you will have to discuss the issue of "great grandma" and the issues you have. I agree that separate living/rooming accomodations are a must for your family, which you should be willing to cover. I don't think it's for you to say IF great grandma comes along - that's their call since she is their responsibility - it's a package deal.
If your husband discusses FL with them and they still aren't open to it, then let it go. Maybe they can't afford it? Maybe it's too much with great grandma in tow? You need to be willing to say, "We'd love to have you join us," and then leave the decision up to them.
Otherwise, you should be willing to go to FL without them and great grandma. If the reason you are inviting them along to FL and to "split the cost" is because you can't afford it, then maybe you need to reassess the trip idea and go somewhere else where just your immediate family can afford. (Not trying to be judgemental here, just bringing it up in case you rely on your in laws financially for your vacation plans.)
Finally, I think if you "give in" and go to the Dells to keep your in laws happy, you will be miserable. Stand up for your family and go with the best decision for all of you. If your in laws are terribly upset, just tell them, "we're sorry you feel that way", and let it go. It is THEIR choice to get upset. Not your fault for disappointing them.
Best of luck. I hope it goes well.