I always had the same idea about a 5th wheel as others mentioned on here.....if you break down, at least you will have a place to sleep. The costs of insurance and upkeep of two engines.
But, as others have said, if you are in a motorhome and break down, that most repair places will let you stay in the mh. As far as costs for two motorized vehicles, I think it really depends. How much you drive a mh and toad is not going to be any more Thant you would drive a truck (w/5er). If you drive a my 10,000 miles a year and a toad 5000 miles a year, that is no difference that driving a truck 15,000 miles year, because its the motor for your trailer and the toad all in one. Upkeep is going to be the same or actually could be less depending on the motors of all said units. Insurance, well that's another story. The insurance on our class c which is valued at 3 times more than our tt was and has a motor is only about $100-125 more a year than the TT was. The truck (f250) that we had to pull the TT was more in insurance. Partially because the fact that the motorhome will be driven less, but the motorhomes value is still more than twice the value of the truck. So I don't really see the cost difference there.
You will or can get more room out of a 5th wheel. But let's face it, when you are electing to live in something that small, is it going to make a huge difference in a few feet? Better designed and use of space is more valuable than actual floor space. You also need to decide how much time you will be spending in it. Are you gonna look at the outdoors from a window or be out in it. A 5er may be rough for your wife with the steps and her knees. Depending on the mh, you might be able to get something with not as many steps. Our TT sat up high like a 5er and that is one thing I don't miss is the many flimsy steps. Our class c is nice about that part. Just a couple low solid steps. Ha e you considered a class c or b? Ours is 32' long in total length but is a 30' model. The front seats are good for storage when camping. Plus a B or C can be worked on by many more mechanics than a motorhome. The downside is they are low to the ground and if you plan on being far off the beaten path, that could be an issue.