I have two in the driveway with truck plates, the state considers the HMMWV a truck, what else could it be? it could be argued a helmet top with windows added to the rear and sides would be a passenger vehicle like a suburban but other than that it is a truck.
I'm going to debate titled body style vs identified body style for registration purposes. It's more flexible than most realize and the state's take on this has shifted of the years. I deal with this literally every week and get calls about military vehicles coming through the DMV as subject matter expert.
I have an official email from a Tarrant County auto theft inspector that an HMMWV body style can be but not limited to a PK = pickup, LL = suburban, SV= SUV, or FB = Flatbed. It all comes down to how the vehicle is setup
and what the owner declares. Example: M1097 2 door with troop seats. Without a cargo cover it's a PK as the troop seats are unsecured (no seatbelt) and unenclosed. If you enclose the rear it's technically a suburban as it has the design and ability to carry more than 4 passengers in a vehicle. This is similar to what you said above but Texas doesn't care as much about the body style unless they can get $$$ from it. As of 2020 they're after weight on titles. If your vehicle's curb weight (empty weight) is over 10,000 lbs it must have a weight slip to accompany a title/registration transfer unless it has it on the title already. They want that heavy vehicle use road tax money!
The rules are vague and over lap quite a bit. The officer had a lot to share about the system and had examples about classic card he was asked to do 68-A (vin inspections) on and he has to declare a body type at that time. Don't get me started on my El Camino/Convertible/Amphibious Gama Goat story which I won against the DMV and Tax Office.
They just want to check boxes and move on so don't get caught up in body types too much.
The VTR-135 (FMV form) had 4 categories for "Vehicle type" and they're broad categories.. but they're how your vehicle is identified. Is your M1097R1 "LL" a "passenger" or "truck"? A passenger.. what? What kind of truck? "Truck" isn't a body style by the VTR-249!! It's all vague because the are trying to catch all body types, styles, and oddities that would fall through the cracks like say.. an M8 Grayhound. Truck or passenger? It's not designed to haul cargo but it's a little too heavy for a passenger status in theory despite designed to move and protect occupants. Now on the title, you could classify it as an AR (Armored vehicle) if you're trying to be painstakingly accurate in your mindset but that's for bank trucks and etc. The state and county will happily slap you with odd regulations and taxes like crazy because it's not what that oddly specific body style was designed for in their eyes. That M8 is now a PK or or LL because it's a best fit for the identified role of the vehicle.
Texas Administrative Code
(4) If the vehicle is registered as a former military vehicle as prescribed by Transportation Code, §504.502, the vehicle's registration number shall be displayed instead of displaying a symbol, tab, or license plate.
(A) Former military vehicle registration numbers shall be displayed on a prominent location on the vehicle in numbers and letters of at least two inches in height.
(B) To the extent possible,
the location and design of the former military vehicle registration number must conform to the vehicle's original military registration number.
Registration numbers should be static to each vehicle but can be missing, illegible, or simply wrong. The state relies on the owners to self report information they find factual to their vehicle or supplement it as true as they can for compliance. Easy example: not having a VIN is non-compliance. If you're unable to locate or find the VIN you can have one issued or make a case to your inspecting officer to have a specific VIN issued in compliance with the original OEM's VIN scheme. They will have to check if that VIN is in existence already just like any VIN inspection but in doing so it's done in "good faith" (huge thing!) to keep the vehicle true to its original form.
All this stuff is fascinating to me but obligatory I'm not an attorney nor do I speak for any parties, organizations, or etc. This is all an accumulation of years od direct dealing with the Texas DMV, dealer's side of things, county officials, and tax offices.