I've dealt with rental companies quite abit, and you usually can't reserve a specific vehicle, unless it is there upon your arrival for car pickup. In other words, you may be able to request a truck, but getting one that is allowed to tow may be another thing altogether.
when my brother's truck broke down (1st day of a 20 day trip), they called every type of rental agency (the usual suspects, driving force, etc). to no avail. The basic response was, we only have a couple of trucks we allow to tow, and they were supposed to be returned awhile ago (weeks). So that tells me that they are VERY hard to get ahold of. Even reserving something, there is no garuntees. A rental company won't leave a car sitting unused just because it is reserved in a week.
If I were you, I'd get a tahoe. From the sounds of it, you have a family of five, and I imagine they have friends that would want to come along at some point.
I have a truck right now, and am very close to pulling the trigger on a 3/4t SUV, while I know there are compromises, I think the SUV will suit our lifestyle better. But I do have access to a truck or utility trailer if absolutely required.
While I understand what everyone is saying about just adding a tonneau/topper to the truck, it is not the same. The bed of a truck, no matter how well you try to seal it, always allows dust in, and it is always hot (even when driving). With an SUV, at least while driving the storage area cools off.
Now to camping, Yes, there is some downsides. one big one for us, is the BBQ. Our Weber Q does not fit in the pass-thru storage on our trailer, and I do not want a smelly BBQ inside an SUV. So we'll either have to change BBQ's or find a good storage solution (tongue rack??).
The other bad thing, is leaving a campsite when it is raining. Right now, we could just throw everything (wet) into the back of the truck and go home and worry about it there. I'll probably end up with some tupperware containers for this purpose.
At any rate, you have to weigh the pros and cons.