Class A RV battery drains while pulling tow vehicle

Perhaps your alternator isn't putting out a proper charge.

What happens when not towing?
It could be an issue with MH to toad connection. Is there a charge line going to toad?
Yes, it could be a weak or failing alternator.

If you have the (-) phase connection off, on the toad battery, your MH battery may be powering the toad and your MH. That may be more than the alternator delivers or may indicate a weak MH battery.

When you're parking for the night, I'd suggest to connect the toad battery back together and run the MH for a bit. That will charge the toad battery and you will then be towing with two batteries.

Thanks for all the quick responses. When the RV is not towing or when in storage, it starts quickly.
Would that be the case with a bad alternator?

Not knowing what type of connector is mounted on your RV and how it is wired, makes diagnosis very difficult. if you have a four wire connector on the towed and the RV, there isn't much the towed can do to drain your RV battery.

My Bounder had a 125amp alternator and that will run a lot of stuff if yours is close to that. I'd check to se if there is something causing the charge control of the RV to open when you're towing. Maybe it's time for a trusted mechanic to check your alternator and charge control circuits.

Oh no! The dreaded double post syndrome, sorry.

Alternator could still be weak.
What type of umbilical cord do you have connecting the toad?
I am still thinking problem in toad connection.
It could be a combination of issues. Maybe weak batteries in coach along with draw from toad.

Also - check out the status of your coach batteries - when your driving the alternator is trying to charge both sets of batteries and if your coach batteries are in bad shape the alternator may not be sufficient.

I would have the alternator and both sets of batteries checked out - most places will do for free.

Toad not wired up to the coach. I use magnetic lights on the roof, 4 pin connected to RV.
Will check batteries and alternator. Thanks

Are you using an adapter on coach or is a straight flat four?
Sounds more and more like bad coach batteries.