Best Case Scenario:
You hire a good photographer and put your house on the market for an outrageous price, like $325k, and out of the blue someone gives you $310k for it.
You invest the tax-free proceeds in VTSAX and earn an average 5% per year for the next 3 years, with a lot of nail-biting choppiness.
Meanwhile, mortgage rates increase from <3% to 5.5%, popping the real estate bubble. Your old house is now worth $220k.
After the van life gets old in year 4, you buy a similar house for $220k in cash.
Your investments have done much better than housing, you've had a great adventure, and you made $90k inadvertantly timing the second housing bubble.
Worst Case Scenario:
You hire a good photographer and put your house on the market for an outrageous price, like $325k, but it does not sell.
Meanwhile your plans and dreams are on hold. You drop the price to $300k. Still no bites. $280k and now showings...
You put the funds earmarked for college in a bond fund like TLT while you wait so that it earns some interest.
Interest rates rise. Your house is now worth $220k, your bonds just lost 25%, and the stock market is volatile.
You take your house off the market because you're feeling like hunkering down.
Several years later you're back where you started but never did get around to having the adventure of a lifetime because of that house and a routine change in interest rates.
Worst than Worst Scenario:
You quickly sell your house for $280k and buy a $70k van.
Housing prices continue to skyrocket, and your old place goes up to $370k in the two years it takes for van life to get old.
You buy a similar house for $370k.
You lost $90k to housing price appreciation, plus the van depreciated by $50k.
Then the housing bubble bursts and the house you bought for $370k goes back down to $280k.