FWIW the handgunlaw.us PDF says nothing about motorcycles. Really! Check it again.
FWIW § 541.201 of the TX Transportation Code defines passenger cars, various types of buses, trucks, truck tractors (i.e. 18-wheelers), farm tractors, mopeds, and motorcycles as motor vehicles. However, IMHO the phrase "inside of..." in § 46.02(a)(2) seems to effectively disallow carry on a motor vehicle without an enclosed cabin.
I'm still confused, but will err on the side of caution.
confusing. IMHO erring on the side of caution is a wise choice until this issue is legally hashed out.