Have to agree with B.O. Plenty...liability would be a big concern with renting a truck and 5er to a novice. There is a "skill set" required to handling that combination that isn't necessary for the Class Cs that you see being rented. U-Haul rents good sized panel trucks to anybody and those small Class Cs are no harder to handle. A renter might hop a curb, but, for the most part if the hole is big enough to drive through they are good to go and backing up is not a lot more complicated.
As for buying a 5er and not liking it? You've got a TT...why get a 5er? You want more room? You want more storage? You want even more comfort? Then you get a 5th wheel. You get more of everything in an effectively shorter package. No tongue space and some of the trailer's length is in the bed of the truck. A little learning curve on backing, but, if you are fairly good with a TT of some length, a 5er is easy enough to get the hang of.
If you don't want more space, more comfort, easier tow, then you must be satisfied with your TT...don't get a 5er just to keep up with the "Joneses", but, if you have a valid reason for being dissatisfied with a TT, then getting a 5er is the next step if you want to keep towing an RV. We wanted more space, more storage and more comfort, but, I didn't want to tow a 30'+ travel trailer and still not have enough space. We got a 5er and have zero regrets, just smiles. If you go with a 5er, you must get an adequate truck to pull it with. I've only met a couple of people that ever bought a 5th wheel and regretted it; one didn't have enough truck and the other wished he gone whole hog for a DP.