Our Dometic RM3663 12v heater element is 235 watt, or 16-19 amps depending on voltage. Some are larger, many smaller.
On a TC refer I would verify what DC element your trying to power.
30 amp draw, assuming your only charging at 12.5v, would be like a 375 watt element? A normal charge rate though @13+v would need like 400 watt heater to get a 30 amp draw?
Obviously there are other needs (battery charge etc) but, usually nice to start with real numbers...
IF your going to add an auxiliary charge line my first recommendation is reconsidering adding a dedicated additional plug. That is where your limiting yourself from the get-go. Plus you need the equal size ground.
The second relay to energize solenoid not needed-fuse on key-on source sufficient. Could though its yours, just an additional draw, albeit small. The ability to disengage solenoid/aux charge line, (switch on energizing feed) is a good idea.
