First off, welcome aboard.
You do sound like an incurable romantic, for which your wife should be grateful.
Not many of us left around.
Next, top me it sounds more like an "the right tool for the job" equation.
Not that it can't or hasn't been done, but the Great Loop and Cruising the Caribe, seems like two different kinds of cruising with differing requirements for each boat.
If you want to do the loop first, do the loop.
See how well the bride acclimates to longer periods underway in a smaller vessel?
Once that's done, if you want to head to the Caribbean or ????, find something that suits your needs (size, comfort, safety, range, etc.) and upgrade.
As far as the truck, you really don't "need" a diesel to pull most things under 28'. Most big block gassers will do just fine. The problem is, the fuel efficiency. While nothing will be "efficient" in this situation, the gasser will probably double the consumption or close to it, to do the same job. Now you have to factor in the cost of fuel in your area and see where it ends up.
The diesel will do the job with less effort than the gasser, and engine life will likely be double that of most gassers and then some. That said, there is more maintenance on a diesel than a gas engine, so additional cost.
In the end, it's all a trade off.
How often and how far are you going to tow?
What weight will you be towing?
Flat land or mountains?
If you can round up an older 7.3 diesel or the dodge as mentioned above, the price tag "should" be more reasonable, and both will do the job admirably. May not be the best looking things on the road, but you never know, and if you're handy, body work isn't all that bad.
My buddy bought a 7.3 Ford quad cab in very nice condition, with less than 160k on the clock, for 9k and change.
The guy used it to tow their travel trailer and that was it.
The last tow he made was to Florida to stay. Said he wasn't leaving again, and if he did, would not be taking the trailer, and no longer needed or wanted the truck.
That rig is probably good for at least 250-300k before even starting to think of doing more than general maintenance. 160k is barely broken in for most diesels. That on most gassers is a much different story.
My buddy's still using it today to pull his boats around and as a DD.
So look around, the deals are out there.
All the best