Been there and done that several times, well sorta. The vehicle was never unrepairable - it all depends on how much you are willing to spend.
Our first long distance trip towing our trailer, the radiator in the tow vehicle started leaking on a Saturday. Was able to make it to a nearby Chevy dealer and they replaced the radiator, made that half of the trip $600 more expensive and about 4 hours longer. That same trip, on the way home, the tranny died. Was able to make it to the nearest campground and then had the tow vehicle towed to a GM dealer. Two days later and $1600 more and we were back on our way home.
We've had several other problems with our tow vehicles. You just have to roll with the punches. Stuff happens, just be as prepared as possible. In every instance (except the leaky radiator since we didn't need their help) AAA was able to bail us out. Whether it was towing trailer and tow vehicle to the campground then dropping the tow vehicle off for repair or just towing both to the house.
We've had AAA for many years in three different states we've lived. Right now we have AAA Plus with RV. We've used it more outside our region and have never had an issue. Sometimes it may delay things because our local region has to connect with the current region we are in.
-Michael