Hi j_rg,
Welcome to the Heartland Owners Forum. There's a lot of useful information here along with a great bunch of friendly and helpful people.
If the dealer installed the batteries incorrectly, they may have blown the Power Converter, but I don't think that explains the problem with the TVs.
If the TVs popped and smoked when the 50Amp cord was plugged in, it seems most likely to me that they were damaged at that time and you are now seeing the full effect of that damage. I'd guess that they still worked after the initial shock, but had a weakened power supply component or fuse.
Smoking the TVs suggests a 110V wiring issue, usually at the power source. After the smoke event, the dealer should have examined the power pedestal and all the 110V wiring to your circuit breaker panel to determine precisely what happened.
If you have generator prep, there's a transfer switch that also needs to be examined for damage, along with the Power Converter. Your microwave may also have taken a non-fatal hit.
So my guess is that this probably had nothing to do with the swap to 6V batteries.
That said, if you have also installed an inverter so the batteries can power 110V devices, maybe it could have been related, depending on how the inverter is installed.