I have a horse trailer that I've outfitted with a large battery to run lights, an electric tongue jack, and even a tire pump and wheel jack. It uses a DC-DC charger to keep the battery topped off when the trailer is connected to the truck.

The trailer has a breakaway battery box with a small battery to engage the electric brakes when the breakaway cable is pulled. The circuitry in the box only deals with showing the battery level and charging the breakaway battery. And to be frank, calling it a charger is an exaggeration. They way these things are built cooks both the battery and circuitry. I end up replacing the battery every year and the breakaway box every few years (depending on how much I'm using the trailer).

I have looked for laws and regulations both federally and for Virginia in regards to trailer breakaways. I can't find anything that demands the trailer breakaway have its own independent power source. I have seen people cite that the breakaway is supposed to be able to hold the brakes on for 15 minutes but I haven't even found a regulation stating that.

I'd like to tie the breakaway system into the main trailer battery as I feel that is a much more reliable and trouble-free power source. I can't think of a reason not to and I can't find any regulations that say I can't.

Am I missing something or an I good to proceed?

That's what I do on my trailers that have stand alone power. I trust my commercial group 31 more than the little cube battery anyway. Plus if my tack room lights are dead I'll know quick something is up.

Exactly. Plus the DC-DC charger in the trailer can regulate and properly charge the trailer battery. The breakaway controller is just putting a resistor between the little battery and your vehicle's electrical system. Its overcharging the little battery, and when the battery is low or has gone bad, burning up the resistor and circuitry because its not actively regulated.

My travel trailer is wired the same way.  One battery for RV house and trailer breakaway.