I suspect most will agree with RickO on special requirement for large Motorhomes. So I will take the contrarian view, for a brief moment.
By the time most people go to take the test, they may be anxious, but fairly confident they're going to pass the drivers portion of the test. That's the critical part of the testing from my perspective. Given the small number of RV driving schools scattered around the country, most have already put unlicensed, unsupervised miles on their coach somewhere.
If you read this and other threads on the drivers portion of the Texas Class B you will find that what is tested behind the wheel varies greatly by location. Some have to parallel park, some back up 100 feet, some drive around the block and some have to get on the freeway. So even with the behind the wheel you have a variety of "tested" expertise. As for the parallel parking and 100 foot backup - passing it on the test may be the last time you ever do it. Highly unlikely that anyone would do it often enough to remain proficient - and with routine driving experience you will become more competent with such things anyway. You would hopefully have a spotter in such cases anyway. I know some travel solo and would not have a companion spotter, but again - rare event for most - not that difficult for an experienced driver. J
The essential difference between the Class B and regular written drivers test is air bakes and how they work. That's a need to know so that makes sense. But making a two day outing of it in most TX locations - written one day and driving the next or another day is kind of silly. You could just go online (or in person) and take the air break test, pay a small fee and your current license amended and sent to you with the same expiration date.
As for going full CDL - you always have that option. The behind the wheel is the same but the written contains parts that most won't need.
So that's the contrarian view. My personal view is pretty close to that but it is what it is. Make sure you're properly licensed in your state and you're good to go anywhere. I know there are some who would object to a Federal Law being imposed on driving their Motorhome and some would support it. YMMV
Wonder how all those farm boys learn to drive combines and every other kind a big thing under the sun without a parallel parking test. Ok - I know. Running over a scarecrow isn't a risk to anyone, but there are plenty of bad drivers with a license, so passing a test doesn't guarantee much.
No substitute for experience. As little as some drive - they relearn every time they get behind the wheel.