Seems to me it's not a commercial vehicle if you personally own it and don't use it for commercial, or more specifically money-earning business ventures or purposes outside of your own personal transportation. I'd be enclined to contact a lawyer and do like a couple other members did and bury them in legal papers from the law firm of "Don't tell me what to drive or I'll sue you for infringing on my constitutional rights as an American who payed a boatload of money for my own home Esq.". On the other hand, if your truck insults thier dilicate sensibilities that much, tell the HOA they can buy your house for current fair market value, pay for your moving expenses to move to another neighborhood without nose-pickers who have no business of thier own and have to be all up everyone else's Kool Aid!
I'm no real estate professional, but that looks like at least a million dollar home, and with that kind of investment should include having the right to park whatever YOU choose to drive in YOUR driveway that came with YOUR house!
Along with the fact that YOUR truck has an American flag flying from it, tells me whoever complained must be an un-American anti-freedom stinking commie!!!!!
Knee-jerk anger-based reaction aside, are you up on the local ordinances regarding what you can and can't have in your driveway? If the local laws and codes are clear and you're not in violation, I'd be enclined to point out that your truck is not a commercial vehicle and what you do with it is up to you and you only. Furthermore, any further letters will be construed as harassment and subject to legal action by any of 3-dozen ambulance chasers who'll be more than happy to slaughter another forrest to bury them in legal papers.
Yeah sure you catch more flies with honey than vinegar, and you always wanna start out as polite and respectful as possible, but sometimes those with control issues take that as someone they can bully and walk over. At that point, I'm ready to give them the "David Banner speech" which sounds like this: "I'm trying to be reasonable, but don't make me angry. You wouldn't like me when I'm angry!". Sometimes just telling a code enforcement or HOA nose-picker that it's not a commercial vehicle isn't enough, and takes such information on a lawyer's letterhead for it to sink in and be understood.
Good luck dealing with the fascists and let us all know how it goes.