Having owned a 15 footer (stickie) as my first trailer, then having (briefly) a 13
Boler, followed by a
Boler 17 and now a
Bigfoot 21, I gotta say that for my wife & I, bigger is ALWAYS better - no exceptions! (he says, casting covetous eyes at a
Bigfoot 25!)
My son has a
Surfside 14, as does a buddy who simply cannot fit into a
Boler 13 (he borrowed mine a couple of times when I had it - but he is 6 foot 4 inches tall! However, that DID get him out of a tent and into his Surfside) The difference that an extra foot makes is unbelievable, and then, when you see the incredible job Tammy & Reace do with a 15 footer which feels more like a 17 inside.......
People are speculating about what will happen as fuel prices rise even more. My prediction: It will be the 13 footers that will lose value faster that the larger ones.
If fuel economy is overwhelmingly critical to a decision, then a substantial price rise will simply result in a decision NOT to go camping at all! (Blasphemy!

)
I do not think the guys with the 40 foot Moho's with umpteen dozen slides, all the gizmos and a luxo car on a trailer behind are gonna be bothered by fuel price - if you can lay down a million bux for a moho, the cost of gas (diesel fuel) is not even a consideration!
My guess - you can quote me and laugh at me a year from now if I blow it - but:
FGRV owners are not the typical campers. There are a zillion different reasons for choosing a FGRV.
Economy is one reason. If it becomes uneconomical, the decision may be revisited, length and frequency of trips may be cut back.
For spouse & I, we have never in our lives "gone camping" (OK -once about 30 years ago, with a truck-mounted camper - we went to a Nat'l park for a weekend. We arrived on Fri eve, and by Saturday early afternoon, we said - "this is nutz, I hate this", and we went home!) When we go to the NOG in April, it will be our first "camping trip" - it may well be our last! We use ours to bring a convenient bedroom along for attending events. Fuel costs would have to prevent our attendance at the events at all before it would impact our usage.
Others are at the opposite end of the scale - they travel a lot of miles to go
boondocking. For some, this is such an important component of their lives that they may downsize but never consider missing a chance. This group will be the core of the continued use of the 13 footers.
Some use the trailers for family vacations - my prediction: Vacations will be shortened or a different vacation mode will be used. Fewer trips will be taken.
My prediction is that people will continue to buy what they are comfortable with - in terms of how nice it feels inside, how nice it feels to tow and the level of financial comfort in terms of purchase price and cost of usage. I suspect that the financial "comfort" will be more of a factor at the smaller end of the market than at the "million dollars each" end, and if fuel prices go WAY up - there will be a lot more campsites available!
We'll see what happens.......