STEVE220,
You're not alone in this situation. We have a very similar operation/feeling in our '04 Itasca Horizon, 36GD with the C-7 330HP CAT. It feels spongy when you step on it. I've been a welder most of my life and, have checked the structural attachment points on this setup, more than a few times.
The steps are mounted to basically the body of the coach, with some structural support. But, it's just not as good as some of the steps on friends motor homes. Some of those are like stepping on concrete steps. Now, not having been underneath yours, I cannot say for sure, it's mounted just like mine. I'd have to see the mounting system and, observe just how much it flexes, when someone steps on it.
That's what you need to do, if you haven't already. Observe all the attachment points and make sure that, there's nothing "giving" more than it should.
Now, because there's really not much I can do on mine, to bolster the mounting system to make it stronger, I simply use what's called a "Step saver". They're sold by Camping World and other RV parts/equipment suppliers. If you haven't seen them, they're simply a double plate, with a welded set of threads, male and female attached to them.
You get to your camp site, set up your rig, do what's needed to level it, put out the slides and all that. Then, when you're all done, you put this step saver under your step and, adjust it so that it's just tight. Your step will now feel like you're walking on concrete steps.
But, there's precautions to take. First off, you need something as a warning to yourself that, you need to remove that step saver prior to getting ready to leave camp. Otherwise, you could damage not only the step, but, the body etc, where it's attached if you were to leave it under there and start the procedures for getting ready to break camp.
Likewise at home, if you use it there. Anyway, enough blabbing for now. Hope this info helps some. Good luck
Scott