What you own today is model 2351. I personally love that model. We have owned our 2350 for 10 years and love it, but a 2351 with all the benefits from the extra foot would make our 2350 better yet.
Now about your concerns with model 2100. I have never driven one nor even walked through one. But from what I have learned with our 2350 along with 2100 owner comments, the 2100 is going to be the worst handling (or should I say "the least favored") model of all Phoenix Cruisers. It comes down to how the house weight and it's contents are distributed between the front and rear tires, and from your question and concerns, you seem to already understand that.
If I were buying a 2100, I think I would do what I had done with our 2007 2350, making the following suspension improvements, all specifically to address handling issues.
- installing a heavy duty rear stabilizer bar (our 2350 had no bar of any kind)
- replacing the stock front stabilizer bar with a heavy duty one
- replacing the shocks with heavy duty ones specifically for a motor home.
- adding a rear trac bar (this eliminates "Tail Wagging")
- replacing the stock steering stabilizer with a heavy duty one specifically for a motor home.
I did not implement my next suggestion into our 2350 but feel it properly addresses the issue with the rear end sagging. I feel every 2100 would benefit from one or two extra leaf springs for each rear corner to lift the rear to the proper height. If I increased the height of our 2350, I fear it would not fit in our garage, so we live with our slight rear end sag. As for you, adding rear air bags as others have done will lift the rear end as well, but I feel increasing the stack of leaf springs is addressing the matter properly and most reliably.
For people who are not mechanically inclined, I also recommend going to the right shop to get the work done. We took our 2350 to THIS
shop, shared to give you an idea of the type of shop I am suggesting.
Once everything is completed, I would then have a front wheel alignment with the rig loaded up with water, gear, food, even added weight to represent people and pets. A wheel alignment is strongly influenced by the weight of the house and contents, and the distribution of their weight. You want the rig to represent a typical trip weight (and weight distribution) during the alignment process. The shop I suggested made a special modification to the front suspension to get it aligned properly. They installed off-set bushings. The topic of a front end alignment on a rig with poor weight distribution is it's own discussion.
I see it's your first post. Welcome!