Come on, is this just a gas usage, gas cost issue really?
Every summer folks panic about the high gas costs of driving lo mpg recreation vehicles.
But my experience is that, once you have already bought and owned your big gasser, the cost savings to switch to and maintain a diesel are false, if you wind up paying lots more for the switch than you already have in your rig.
I bought a 2004 Chevy HD camper van with the 6.0 v-8 gasser (for much cheaper up front cost than getting a diesel) about four years ago. I've put about 40,000 miles on it since then, taking two big trips a year all around the NW and then the SE for the winters. It has tons of power, can accelerate madly on hwy merge ramps, hauls a huge pile of gear (gone for up to 4 months at a time), is quiet and not smelly, and gets a shitty combined 12.9 mpg on these trips....
But.....I got it for about $22,500 when a diesel would have cost me around $10-12k more. I have not spent 10k$ more on gas or maintenance in these past four years.
Powerful, yeah; quiet, yeah; mpg, no.... But it works for me.
So, if you already know all of the good and bad of your current van, and can figure out how to improve it for your growing needs, why spend your hard earned money to buy someone else's headache?