Our slides only work with the key in the Off position.

Our only work with the key in the on position

2019 45 OPP

2019 40IH. Ignition key must be in the on position. Engine does not have to be running. Generator should be running or hooked to shore power.
jt

Which bring up the OTHER issue/ reason: (MOST) now say LEVEL first, then extend slides. On MANY, if IGN=ON, then Jacks= AUTO RETRACT to PREVENT "Drive-off damage".

Auto Retract, at least on my 2008, meant that when activated, no need to hold the button down and all 4 Jacks would retract. If you would put it in gear and start to drive off, no way the Jacks would retract fast enough to prevent damage.

As a few posters have noted, it is strongly recommended that diesel engines have a 3 to 5 minute cool down period and a 3 to 5 minute warm up period.
That is the perfect time to address your slides and jacks.
Arrive: park, get my 50 amp cable out, not connect, hit the auto-level, which dumps air, levels and stabilizes, extend slides, connect power, shutdown the engine.
Engine cooldown achieved.
Depart: all inside travel prep completed. Water and sewer disconnected. Pre-trip checks completed. Start engine for warm up. Retract all slides. Hit "Store" on jack system. Disconnect shore power and put cable away. Hit the road. Engine warm up achieved.
Note: even though the manuals for many of the Tiffin REDs state ignition needs to be "on" when operating slides, that is not intended to dictate that engine is not running. When operating the slides and jacks, the engine can be running, or not running if you already achieved the important warm up or cool down.
One other note: I wish there was a way to dial back the volume of the loud beeping that occurs the entire time the key is on and the air is dumped, or pressuring up. The pets absolutely HATE this. Does anyone know where the beeper is located? Maybe I can muffle it?

I would think the original poster, FredT, who appears to be new to owning a larger RV, is totally confused at this point. Documentation in the manuals conflicting, and many of us seem to have different approaches. Think most of us appear to agree that the slide motors pull a lot of power. Key here is to supplement your batteries wherever possible. Some prefer shore power or generators, others with the motor running. The one reason I always used shore power was concerns that the air bags might try and inflate again and to keep noise down to a minimum. After years of using shore power, I read somewhere that the system in my Phaeton had a lockout that prevented the air bags from reinflating when the Jacks were extended. Never dug into it further since using shore power seemed to work. If dry camping somewhere without shore power, would definitely suggest poster to use the motor or generator to prevent unnecessary drain on the batteries when possible.

Back to the OP question, all 2016 or so Tiffins and older do not have a factory installed BIRD. They just have a solenoid that allows you to combine the 2 battery banks as long as you hold the boost switch down.

Therefore, running the engine will have no impact on the house batteries (which drive the slides), if you don't have the boost switch down.

However, if you are plugged in, the inverter/charger will help the house batteries while you are operating the slides.

I've always followed the warm up/cool down the engine while you work the slides and jacks. After my test to confirm they are powered by the house batteries, I'll just be sure I disconnect the shore power after those 2 steps.