Usually camper trailer (travel trailer) lengths are counted using the "box", or livable area. So my 21 foot TT may be 25ish feet overall, but is still considered a "21 foot" TT.
Simply put, the shorter the trailer the tighter you can maneuver. Longer trailers are, in general, easier to maneuver / back up, but you need more room. Once you are on the highway you really won't notice that much difference.
Until you get used to towing and know the area you will be in, plan on using highway gas stations to refill - they should have plenty of room. Highway gas stations will vary, but in general if you just need to refuel with regular gas you will use the car pumps in the front. The truck pumps in back are really for the big rigs (tractor trailers) and larger campers / motorhomes, but there usually will be a gas pump back there if you end up back there. For parking you usually will be with the big rigs. If you are parking in angled spots, try to park so the rear of your trailer is visible to approaching trucks so they know the spot is taken (you may see what I mean when you are parking). I try to stay out of the way of the big rigs - they are working and I am on vacation, I'll take the far spots in the lot.
Your may find your engine revving higher than you are used to while towing. Put the transmission in "tow/haul" (or whatever setting is appropriate for towing) and let it choose the gears. On the highway the transmission will probably shift into high gear (overdrive) eventually when on straight and smooth stretches, but it may take longer to shift into than you'd think. You'll figure out how your engine and transmission works. I use cruise control when towing on straight and smooth sections, makes it easier for me. When needed I turn off cruise control - hills, curves, traffic, etc.
If you have a chance, try changing a tire on the camper at your house. Figure out how you will lift the camper (up on ramps, a jack, etc). I found that the tire changing jack in my truck will work to change tires. Make sure you have the correct socket / wrench / socket wrench - I found that my truck's lug wrench does not fit the trailer tires, so I made sure I have a proper size socket and socket wrench in the trailer. If you have AAA, double check to see if the service covers towing the camper or something like changing a tire on a camper - the plans and regions are different.
Go camping, have fun, have a credit card handy and a good sense of humor, and let us know how it goes!
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2021 F150 2.7
2004 21' Forest River Surveyor