Electrical problem

The relays that disconnect the house and chassis batteries are 'latching' type relays that will stay closed until they are electrically 'unlatched'. Sounds like your engine (chassis) battery disconnect relay has failed. I doubt the boost relay welded unless you were just using it to 'boost' a dead chassis battery to start the MH. Another possibility is the switch for the disconnect has failed and is no longer delivering the 'unlatch' voltage to the relay. You need to find the relay and jump 12 volts directly to the correct terminal to see if it will disconnect. Get the numbers off the relay and look for it online to determine which terminal you need to jump to (never put jumpers across things you aren't sure of unless you like burned fingers and seeing how good your comprehensive insurance policy is). As to finding the relay, have someone work the switch for the house battery while you nose around looking for where the clicking sound is coming from. MOST of the time the two relays are located together. Expect to pay around $75 online for a replacement if the relay has failed.