I drive in the highest gear that will let me keep the car moving. If I'm just cruising, I can use 5th as low as 30mph. I like to down-shift before the car gets to 100% load (WOT) because I don't want it going open loop and running slightly rich if I can avoid it. So if your foot is on the floor, you have chosen a gear that is too high. But I don't think it's possible to do damage to the engine or drivetrain from having the RPMs too low.
The only caveat is that you may burn more fuel by accelerating in way too high of a gear with the engine turning too slowly vs. one gear lower and allowing the engine to run where it makes power more efficiently. That's mainly because you could end up trying to accelerate forever rather than getting the job done and backing off to a steady cruise.
Also keep in mind that driving in a very high gear makes fuel economy EXTREMELY sensitive to throttle position. It's going to be difficult without any kind of instrumentation, but you need to take care to make sure that your foot really is using the lightest touch possible to maintain a given speed. This could mean the difference between 25mpg and 50mpg at any given moment with little or no seat of the pants indication.
brick you said it all! just remember to listen to your engine... and you can tell its lugging if it doesn't purrs but it sounds rough and you feel more vibriation than ever... but ya light feather throttle its almost impossible to lug unless you start pressing the throttle some more... if you need the juice just quickly downshift one gear since the difference is very little at that rpm. Then shift back once the road steadies.
If your reading this, then good for you, your saving some gas because your here.
I'll second everything brick said. I'd prolly be in 5th at 25 in your car,
I quite often cruise at 25 in 5th. I can even accelerate to some degree as long as I'm not in a hurry. The lowest I have ever cruised in 5th is 20; no acceleration though. My engine is one that has a bore that is smaller than the stroke which makes for very good torque.
Horsepower is how hard you hit the wall, torque is how much of the wall you take with you.
no gas pedal in 5th at 16-20 mph gets me 70-80 mpg 800rpm but the lowest speed I can give it any gas is about 20mph some where around 1000rpm. Too slow an rpm and the oil spray on the underside of the pistons will not be enough to cool the pistons and you start cooking things. MPG indicator getting too low is an indicator of too low an rpm for the load also the power pulses at low rpm can damage the gears in the tranny because the springs in the clutch are bottoming out and that occurs when you get chattering which is bad!! Heavy loads also exposes the exhost valve to longer pulses of hot gasses before they close and cool off. Timing also gets really retarded with the lightest of throttle opening at low rpm.