The OP asked for alternatives, I posted one that's readily available and half the price. The Red Dot R-6101 is advertised as an on-highway unit for trucks. Though, I can't find any documentation to back that up...
Anyone who has spent time on forums has seen something, somewhere that's posted which overtime becomes fact. I would speculate that's where the 30k BTU originates. The Acela trucks claim to have a 30k BTU air conditioner, it looks like the same cab upgrade regularly found and referred to by the OP. If I'm wrong, let me know. Who has tried 20k BTU and thought it was insufficient? Why not 40k BTU? Where does the magical 30 come from?
To address a few posts above - you can't compare house and vehicle AC. Most cars do have 2+ tons of cooling. I'm no HVAC expert but I'd guess that's due to a few reasons. Your house is well insulated and without turning on the AC probably sees a max delta temperature from outside to inside of 15°(?). A car in the summer with the windows up could see 70° or more (?) difference. On top of that, you expect the home air conditioner to maintain a temperature 24/7 whereas a car, you hop into a 130° interior and expect a meat locker before you leave the driveway.
The math is above my head to calculate how much cooling capacity would be required to simply maintain a x-delta temp, however, anything more would be cooling the cab as a function of time. So, isn't the question really, how much time is acceptable to you to cool the cab from 130° to 70°? Ex. You can lower the temp in the cab to a comfortable temp in 20 mins for $3-6k with 30k BTU or 30 mins for $1800 with 20k BTU. *shrug
As an aside, if you're going to the trouble to add AC, why wouldn't you insulate and likely add tint? For this discussion, can we just assume the cab is insulated to some degree?