This is very true. Some go right to the biggest diesel pushers or 5th wheel trailer they can find thinking "I'll have room for everything I'll ever need" only to discover that they can't take it anywhere because it won't fit. Many campgrounds can't handle anything over 30 feet long due to tight turns and limited space. I've watched a diesel pusher, due to it's weight, sink into blacktop when supported by it's auto leveling struts.
The biggest thing to think about is "What do I really need to take with me when I hit the road?"
Funniest thing I've seen is a couple that had gone out and bought a 40 foot 5th wheel, a new one ton truck to pull it with and had never had an RV before. The wife and I had arrived at the campground 45 minutes earlier and had already set up our site and were eating lunch. The "rookie couple" pulled into the site next to ours, got out of their truck and started trying to figure out what to do next. To say it was a comedy of errors would be an understatement. The sequence off events went something like this:
1) Pulled into the site, leaving the rear 6-8 feet still in a drive area. Rookie wife asked husband if she could run out the slides. I went out to help them out
2) Had rookie husband pull rig ahead to clear drive area, asked if he had anything to level it with, met with blank stare and "What do you mean by level it?" Rookie Wife asks again, "Can I run the slides out?"
3) My wife brought out a small level that I placed on the floor of the new rig, found it was almost a "bubble" low to the right. Rookie husband found some interlocking leveling blocks in a storage compartment, showed him how to level the rig. Rookie wife asks again, showing frustration, "Can I run the slides out?"
4) I told rookie husband to disconnect the truck(it was blocking the drive area in at the other end of the site), only to have him start to do so without extending landing gear first. Stopped him from dropping the trailer on the ground and helped him unhitch the trailer. His wife asks again "Can I run the slides out yet?" now getting seriously aggravated
5) I asked the rookie husband if he knew how to hook up the trailer to power/water/sewer? Rookie husband tells me he can't hook up to water as it will over fill his water tank and shore power will damage his batteries, both not true. Explain to him how the trailer is set up and how to hook everything up. Rookie wife decides she's not going to wait any longer and runs out slides, forcing rookie husband to have to crawl under slides to hook up trailer to power/water/sewer connections.
6) Rookie husband finishes hooking up utilities but has the sewer hose laying on the ground, meaning it won't drain properly. I show him a support I have for my sewer line, only to find he had bought one but didn't know what it was for. Installed the support for him as he watched
7) Rookie wife asks rookie husband to set up her "kitchen". My wife and I look at each other confused as the kitchen should be set up 'inside the trailer". Come to find out, they had been watching You Tube videos on what they would need. EVERY STORAGE AREA WAS RAMMED FULL OF STUFF!!!!!!!! Wife and I watched as he pulled multiple camp carpets, multiple tables, multiple camp stoves, electric coffee pots, several folding chairs and who knows what all else out of the various storage areas. Two hours later, he's finally done and I, being the helpful person I am, take a walk around their trailer. With all the stuff he had pulled out, most of the storage areas were still full. When I asked why they had so much stuff, I was told that the internet said they would need it all for a month long trip. At this point, there wasn't much I more I could do except give him the following advice; You need to do two things when you get home:
1) Unload EVERYTHING you have in the storage compartments and only put back the things you actually used this trip
2) Take another SHORT trip, and see what you use or miss on that trip. Anything you don't miss that was left at home or wasn't used on the short trip doesn't go back in to the trailer as all that weight is hard on the truck and trailer

Just got to thinking, that was back in 2014, at a KOA in Spokane Washington. I wonder how much they took out of the trailer and, more importantly, are they still married? All she could think about was running out the slides and her own comfort. If things didn't change, I'd be willing to bet they either don't still have the rig or are divorced because of it