Where in Texas do you plan on registering your Volvo? Counties that have some experience make it a piece of cake, while other counties (or certain Assessor/Collector's offices in those counties) that have never handled a conversion will simply default to telling you that "it can't be done", even though Texas is very open-minded to the concept. Many folks, for that reason, choose to use an Escapees address and go to Livingston County to do their registration, since they've done many of them and are familiar with the process.
Steve from SoCal has given you a good list of things that will qualify a tractor with a sleeper to register as a motorhome in Texas, and as in Kansas, you'll need four of them (unless the rules have changed). When I did it almost fifteen years ago I took the paperwork and photographs of my truck and the qualifying items (a microwave, portapotty, refrigerator, and both the bed and the HVAC controls in the sleeper) to the nearest Assessor/Collector's office. Their initial reaction was, predictably, "No, that can't be done", but after calling Austin they found out that it could, and I walked out of their office with my tags in hand.
It's been too many years since I did mine, so I hesitate to tell you exactly what you need today to do it, but the Texas page of the Resource Guide is most likely correct, as there are a lot of Texas-based HDT RVers out there, and the state's requirements have always been well documented. I do know that some years back a change from a truck tractor to a motorhome required applying for a branded (Reconstructed) title, which was not the case when I did mine in 2009. The form required to do so should be listed in the Resource Guide, and Texas' motor vehicle-related forms are readily available on the web.