Here is my opinion.
I have run the gambit . first was a small camper , then a trailer , than a MH and now a full blown expedition vehicle.
The advantage of truck and camper is irreplaceable.
I always tow. Either a boat or my ATV's or both.
This fact eliminates the camp trailer.
The MH was a pain to maintain and not a great tow vehicle. ( launching a boat with a MH can be sketchy ).
It was a fuel eater.
And it was big. No parallel parking in small towns along the way.
No impromptu side road trips on unimproved roads etc.
I opted back to the TC.
Now everyone knows that loading and unloading a tc IS A HUGE PAIN.
So I decided that a dedicated truck for the TC was a good idea.
No more loading and unloading issues, and it is always ready to go at the drop of a hat.
Now to your question.
A short bed is a poor hauler for even a light TC. ( The cg is just all wrong.)
A long bed is a much better choice.
Going as big as one can on a TC is ( IMHO ) a bad idea.
Get only what you need and nothing more.
If you camp and or travel as we do, you won't be spending any more time in the coach than you have to. That's why God made camp fires.....
Slide outs add complexity, weight and reliability issues. I shy away from all of these.
And lastly. STUDY < STUDY < STUDY.....Before you buy.
Not all TC's are created equally.
Place your priorities highest on construction, not amenities.
To the truck. I like Ford. In stock format ( no mod's ) they just seem to haul better than the other options.
I run gas. Easier to find fuel on the road and less expensive to maintain over the life of the rig.
Mileage has never been an issue for me as the truck is not my DD, it simply hauls the coach and gear.
I get 15mpg with my Ford F-250 BB loaded or empty. ( it is always well tuned ).
My new rig is equipped with a small block and manual transmission.
As I haven't had time to truly travel much in it I can only say that on it's shake down trip, It managed 18mpg over all .
Not bad considering that we were in the coast range of northern Oregon the entire trip.
We were hauling 2 quads on a snow mobile trailer with all of my gear on board the TC.
I hope this helps.