If you are transporting other people, make sure you buy something that has seatbelts in the non-vehicle part of the rig. Class C RVs would tend to not give you as much interaction with passengers while driving, particularly with the smaller ones.
Many people think they need to have a generator on board, but it depends on what you are going to be doing and where you are going to be RV-ing. If you are only parking where there are full hookups, a generator is unnecessary and a maintenance item. If you are going to be camping and don’t want to run a TV or a microwave or toaster, a generator is useless. If you need to run an air conditioner in the coach, a generator is required (assuming you’re not in an RV park). I’ve used mine once.
Good water and holding tank capacities are a plus. The only drawback to my little rig is that the tanks are small, so I have to dump them every two days or less, depending on water usage. On the other hand, most of my trips are only weekenders. On the plus side, I get 14 mpg.
The other problem is material storage. Many RVs have a “cellar” for things like coolers, chairs, etc. This can be critical, particularly for long trips.
Don’t buy something huge that you don’t need. People generally use the RV a lot the first year or two and then usuage drops off dramatically. Then you have a $200,000 guilt trip sitting in your driveway, the resale value of which is not good.
I would recommend that you go ahead and rent several models to see what layout is best for you and which features you like most.
The advantage to a trailer or 5th wheel, of course, is that you can park it and then drive your vehicle around for tourist type activities and not have to tow a car behind your RV (it’s not the towing, it’s the backing up). The disadvantage is that you need to buy a truck that is big enough to pull it and has the right weight ratio, so you may not be saving a lot of money.