It's mechanical solenoid so it will definitely be inexpensive. Why not just use a mechanical relay that gets energized with the ignition and get de-energized when you shut the RV off. Then apply your solar charger output to your house bank. The issue with the voltage sensing relays is they tend to cycle constantly if the batteries approach the voltage where they engage/disengage. Add in the additional resistance of the contacts as it wears and a cheap and easy option makes your head want to explode with the constant slapping open and close of the contacts. Forget that. I go solid-state with the lowest voltage drop I can find. More spendy but more reliable and less aggravation.