The math, in big round fuzzy numbers looks like this;
Seattle to Miami is ~3,500 miles one-way, so 7k round trip, since it's a vacation, let's call it 10k miles.
At 10 mpg or a V10, and 15 mpg for a Sprinter-based diesel chassis and using the same $3.75 / gallon for both gas & diesel that would be 1,000 and 666.67 gallons respectively. In dollars, that's $3,750 dollars in gas, and $2,500 in diesel, a difference of $1,250 on the whole trip.
How much upgrading, and how much space loss do you think is reasonable for $1,250?
BTW, that $1,250 spread out over 6 months is $6.85 a day.