Hi and welcome!
Some of this will echo Manitou's post.
The 21' Chinooks with the Ford V-10 will do well for towing (I sometimes tow a boat (4,500#) or car (3,500# but no tongue weight) with mine, to include steep mountain grades (not just Florida type towing). These would be from 1997-2005. I imagine the 460 Ford engine would do well too (19??-1996). They're not going to compete with something like the Ecoboost for low-RPM torque, but they do the job unless you always want to be the first one up the mountain.
The two possible sticking points are the sleeping three, and the rear axle weight. On the sleeping three I can imagine a few options, especially if it's not all the time.
1) All sleep inside on converted furniture. With the dinette model you can either make basically the whole living room into a bed (couch folded out and dinette folded down), or use it as two beds (couch not folded out makes thin bed, or folded out is about 45" x 72", and dinette folded down is something like 36" x 68"). On the other hand the dinette model is a little cramped, aisle-wise, during the day (but some love it - I like a bit more elbow room so went for the club lounge plan). There are also models with two beds, or couch and bed, but they are rarer. The passenger side is the "optional" that changes for the various plans. It can fairly easily be converted to whatever you want (some have made other plans into two beds, or two couch/beds; changed dinettes to club lounges, club lounges to dinettes, etc.)
There were (in older models) some passenger side couches (with the two couch plan) wherein the backrest hinged up and strapped to the overhead to make another bunk (kinda boat like).
If I were traveling as two people, I'd likely go to the two couches/beds layout. But that all depends on the individuals. The stock couch is about 45" wide folded out. As mentioned above, with the dinette model you can make the whole thing into a big bed (maybe you'd want a topper for the seams).
2) Another option would be for one person to sleep in the aisle on the floor on a Thermarest type pad. Kinda cramped but I did do that in my Chinook while I was re-modeling the couch (I didn't find the stock couch to be all that comfortable to sleep in, so I did some mods).
3) Bring a tent
4) Tow a tiny camping trailer sometimes (Scamp, etc.)
As far as the rear axle weight, that's not necessarily an issue, but just something to know. Reason I say this is if you look at the specs, you see a GVWR of 10,700# and a GCWR of 18,000# So it looks like you could tow a house. But you really can't, because of the rear axle weight rating. It's 7,500# on the E-350 chassis (most 21' Chinooks), and with a loaded Chinook you're probably at 6,800# -7,000# on the rear axle. With the ~300# of tongue weight on my trailer (it's a tandem so I don't have to have a full 10% as tongue weight), plus the physics of the overhang, that adds 550# to my rear axle weight. As you can see, that comes close to topping it out. Not a concern with a 3-4,000# trailer as long as you don't go crazy with weight in the Chinook (but clearly no-one's towing 8,000# without going way out of whack).
If you are considering a Destiny, I would suggest getting scale weights prior to purchasing. Reason I say that is I have read that the Destinys very close to max weight just as built (purportedly that's one reason they started looking into things like foam-coring - the Destiny was so close to maxed out). That could become a problem for towing, especially since there is even more storage and cabinetry right at the back. But if I had my heart set on one, I'd find out for sure by weighing vs. just going by what I'm writing. The Destiny is longer (you get a bigger bathroom and an extra dresser space) and wider (same layouts in the living area but a bit more elbow room). It's on the Chevy chassis.
I'm a boater too, and that's one thing that really appealed to me about the Chinook (and still does, four-and-a-half years after buying it, and after helping friends with projects on many non-molded-fiberglass Class C's). Even a lot of the other ones that look like a molded fiberglass shell are I believe actually put together in sections (Born Free, Dynamax, Bt Cruiser). The Chinook really is fairly boat like in its construction (although there are differences). No seams in the fiberglass (it must have been a real pain to lay them up). There are still the penetrations that you need to keep bedded (roof vents, windows, etc.) but that's impossible to avoid.
BG