Any decent fleet owner should already know about generators and a battery bank, Espar heaters and insulation. But many don't want to spend the money for the long term because they are more interested in getting the van on the road and earning revenue in the short term. Many have also been burned, or heard of others being burned, by drivers who have no taken care of relatively high dollar creature comforts, regardless of how much money they could potentially save.
Also, most fleet owners have a contract that says it's a 60/40 split, with the driver's getting the 60% and paying for fuel, with the owner getting the 40%. They do that because if the driver isn't responsible for fuel, he'll idle like crazy or will deadhead all over the place just for kicks. So, with an owner getting the 40% and not paying for fuel, he really won't save any money by providing money-saving creature comforts. He'll save some money in the long run because the van will last longer if it's not idled a lot, but then again he's only in it to last 5 years anyway, and anything beyond that is icing on the cake and becomes pure profit. As soon as the van needs major repairs, he'll just get another one.
I can tell you one thing for sure, you can make good money as a driver of a straight or big truck, but you will not make much of anything driving a cargo van unless you own it. Best is to get your feet wet for 6 months or a year driving for someone else, saving as much as you can, then buy a good used van, 3 or 4 years old, pay cash for it, put a bunch of money into it for maintenance to get it absolutely road worthy, have at least $5000 in the bank, and then run it paid-for for a year or three while saving up for another one, either another used one or a new one.