I think that there may be some basic misunderstanding here.
The trailer brakes are not operated by the trailer battery. They are operated by the blue wire coming from the brake controller, through the 7-pin connector. The only time that the trailer battery is used for the brakes is if the emergency brake cable is pulled out (usually because of a runaway trailer).
"IF" a person were so inclined to use a Bluetti or similar devise to run the AC and DC power on the camper, it would only require that the person turn off the breaker to the converter, or install a converter shut-off switch. Just plug the trailer's 30A cable directly into your Bluetti.
As far as the tongue battery is concerned, it only needs to be one of these (link below). It does not have to be constantly maintained. These batteries are used in emergency lighting systems all over the world and sometimes only charged ever 6-months or so, just to be sure that they are operational. On a trailer, just wire the emergency brake switch to the tongue battery (nothing else). The battery could be charged before each trip and be good for months. They don't lose charge like wet cell, lead-acid batteries do.
https://www.amazon.com/dp/B07QPYFBW6...NsaWNrPXRydWU=