the need to keep an infrequently needed motorized truck vehicle in ready-to-go shape, not to mention all appliances like AC and generator, as well as preventing leaks and sun damage will take a LOT of effort, time, and money. just look at all the problems discussed in these forums.
so the horse trailer w/ sleeper would be easier to upkeeo, but it needs a truck and you are not close to pups
you said a rental van has worked so far, and this would be easiest cost-effective way to continue, except for those overnighters...
sportsmobile in CA or TX can and does convert a used van into a dog transporter w/ a generator, AC, and likely a bed. this van should be much easier to drive/own/maintain
http://sportsmobile.com/mobile-dogs/