Size Matters???

My opinion is that if you don’t plan to full time in it you can go a bit shorter and concentrate on a floor plan and options that meet your (hers) needs. If you are considering going FT in the near future then perhaps look at something bigger.

I like smaller to fit in more places with less hassle. 34' would be my maximum not minimum.
Depends on where you like to stay also.

We are at the point after 3 decades of RVing with various sizes and classes of RVs, add to that it is now mostly just my DW and I, we went big as we are planning to do extended stays. Appears your new truck is stout enough to handle whatever size rig as even the largest 5ers are being built lighter than ever before.

It is by far our roomiest RV ever @ 40', and yes it might be difficult to get into some of the older RV sites, but we have done so much of that type of camping with our kids it is time to relax and enjoy the spaciousness. We kept our previous RV anyway for those type of trips, although it has been sitting idle for awhile.

You may want to compromise, look at the smallest units you believe you would be comfortable with for long periods.
There are plenty of rigs in the weight and size range you listed that are capable of meeting both requirements. Tank sizes, W/D setups and bathroom areas are really the biggest differences.

What ever you do never forget::: if momma isn't happy no one is happy.

You need to look carefully at the payload rating of your new truck, and the pin weights of the 5ers you are considering. And know the loaded pin weights will be higher than the published empty pin weights. Could be a lot higher if you load it to full cargo carrying capacity. Start getting near the 40’ range and the pin weight may be too much for your SRW. Even some in the 35’ range are pretty heavy. I would definitely looking at the lighter weight models, especially if you want to go long.

People can give advice on carrying capacity and practicality of size, but only you can decide which is more important: size inside, or size outside, i.e., living in it, or parking it. Very personal decision. We went for living inside in comfort. And so far we've never had much of an issue finding someplace to park. We did downsize a little with our latest toyhauler: from 39 feet to 38 feet. :)

I had a small load on my tandem dually gooseneck today and towed it 300 miles and over a couple mountain passes. My gross combined weight was just under 22000 lbs. While I was driving I was thinking “at this weight I don’t think I would know anything is behind me without looking in the mirrors.” I think the new Ford F-350 SRW 7.3 would handle a 16000 lb trailer extremely well. That said, for the occasional camping that my wife and I do we have absolutely no desire for anything larger than our 36’ fiver. Although we do sometimes haul a motorcycle on the back of the fifth wheel.

Edit: I guess I should point out that I have the Powerstroke so I seldom see over 1500 rpm and almost never over 2000 rpm with that kind of load. I would hope your expecting the 7.3 will need to rev a little to make power.

I had a 44' triple axle 5th wheel with the huge slides so it was very roomy inside. But I did have trouble finding nice RV sites for it on short notice. It was 15,700 or so empty and loaded up it was a bear in the mountains. Pin weight was was too much for a SRW truck. Down in Florida I always had to be extra careful to squeeze into those old campgrounds. I usually ended up at the more rustic places inland at monthly rates.

Had a heavy 40' Cedar Creek Day Dreamer that weighed 16K plus for full timing and it wasn't much fun pulling that weight in the mountains either. But it had less overhang and tandem axles that handled good.

I just don't enjoy pulling those heavy suckers long distance, I down sized to a much lighter mid rise 29' for occasional use and its tight inside but easy to move around. If I went back to full time I would stay around 35' or less OAL. I had a 9,800 dry 34'-4" OAL rig and that was a good size for moving around every couple weeks and easy on the truck. I probably should have kept it!

Well weeks or months of travel at a time. We comfortably full time in a 32’ 5er. So several week trips in a 34’ 5er should not be an issue. The 800 miles one way say mostly flat land towing so the 7.3 should not be an issue for power. Likewise with gas engine payload should not be an issue even with a 40’ 16,000# GVWR.
Only consider a diesel if you want to head to either coast with a heavy 5er.