I vote for box truck also.I have an 18' UD diesel box truck with a 17k gvwr.I still run out of room.I have two hot water skids in there,a 550 gal fresh water tank,330 gal recovery tank,10k watt watercooled kohler generator,wash water vacuum recovery and filtration system,stacked pressure,and supply hose reels,and a vacuum hose reel.I have every bit of space utilized,and the walls are covered with shelves and brackets to hold all the wands,lances,surface cleaners,hood cleaning equipment etc.I still won't fill the fresh water tank up all the way due to the water weight,but a box truck is the way to go in my opinion.I also have a 1 ton dodge extended van that I am setting up for just Kitchen exhaust cleaning and air duct cleaning.I got a great deal on the van,that had a lot of money put into it(rebuilt engine,new tranny,starter,alt etc.),and the previous owner went out of business and needed to sell it quick. I had a previous business and went through cube van after cube van,before I bought my first diesel box truck,and would never go back.They are built to haul the weight,the diesel engines far outlast gas engines,and the exhaust brake saves your brakes,which are way more heavy duty than van brakes to begin with.The cabover box trucks drive like a car,are quiet,and your not riding with your equipment hearing it rattle around and smelling the fumes from your equipment and chemicals.The only down side to a cab over box truck,is havingto pull into weigh scales and DOT check points that you usually can drive right by with a standard van.I have also been driving tractor trailers for over 20 years so I am use to dealing with DOT checks,but if you get the wrong DOT guy looking to write a bunch of tickets that day it can be expensive! This past summer I pulled into a Dot check in upstate NY with a tractor trailer,and was inspected and no violations were discovered.Two days later I was pulled into a DOT check in PA (near my home),and a gung ho,I'm going to be promoted to captain in a week type pain in the a$$ DOT inspector, inspected me for over 2 hours ,and wrote me 8 non-sense tickets totalling over $1100 ,he was climbing around in my cab,counting the fuses in my fuse box,borrowed a bottle of windex from me,I thought to clean his goggles,instead he went over every airline and fitting for the air brakes spraying it with windex trying to find an air leak(he found none.).I took the tickets to court and got 5 of the 8 tickets thrown out,but it still cost me $500 and a lost day of work. I found out through a friend of mine who knows that DOT guy ,that he is an overtime hound,and he put's in for all kinds of overtime(he's out inspecting trucks on holidays,weekends etc.),and he writes tons of tickets to justify his overtime,and most out of state companies don't contest them,they just pay them.So as you can see ,DOT inspections are something to think about with the box truck route. I understand that you need it for your personal vehicle also so a box truck really does not fit the bill right now,but if there is any way you could swing it,like maybe buy a cheap personal vehicle to just run around in ,I would go the box truck route.Good luck with what ever you decide.
PS.Don't let my DOT rant deter you from a box truck,that DOT guy was just way over the top,most are not bad if you keep your truck maintained.