P30 Chassis

You might look around for a used Gear Vendor overdrive unit. It worked nicely in the P30 I had..New ones run $2700.
They bolt up to the turbo 400 trans, then all you have to do is shorten the drive shaft- about $120.

I need to find a way to get as close to 10mpg as possible. How much would a gear venders overdrive unit cost in an RV salvage yard?

Check out Gear Vendors online

Ring and pinion, No. You will need the low ratio at some point.
That leaves you two options, newer model transmission with overdrive and aftermarket electrical control or a overdrive auxilliary transmission like a "GearVender".
Both options will be expensive even doing it yourself!

NO YOU CAN NOT. OK maybe you can, but there are some serious considerations, that highly limit what you can do, or what benefit you will get. I'm going to make an assumption that if you are asking this question, that you are talking about a motorhome with a TH400. If so, you have a critical problem - TC stall speed. When I first dropped the 502 in my coach, I changed the gearing, because I had gobs of low end torque. What I found was, that unless the engine was turning 2500 RPM or higher, there was enough TC slip, that it had virtually NO impact on my mileage. So cruising at 55, compared to cruising at 75 or higher resulted in the SAME mileage. It was not enough slip to heat the trans, but just enough to hurt the mileage. The only thing changing the ratio did for me was forced me to cruise at a higher speed. You can just tell, especially with an engine computer and live display what is really going on.
These vehicles are designed surprising well, in that TC slip, gear ratios, and weight are all designed to work together. If you change one thing, there is a domino effect, a whole series of other things that are no longer "in tune" with your change. The picture is bigger than just a personal preference of mileage vs drivability.

BTW: You don't just want to change your TC to a lower stall speed, because then you will never be able to get rolling with the change in gear ratio. I hate to say it, but the only viable solution is going to a 4L85E which will give the acceleration off the line, and the low engine RPM in 4th with lockup. Still not sure that a older 454 will handle it. Maybe a late version Vortec can, but probably not the original 454 from the 70s and 80s.

You can do that, but if you go too far you will find the transmission downshifting to get the revs back up where they need to be, to have the horsepower for the speed you are trying to run.

You won't know how much unless you have the tools and information (engine's power curve, power vs. speed curve for the RV) to do the chassis engineering. The P-30 chassis series came with different rear end gears (in some cases different axles) for different GVWR/GCWRs and applications (RV vs delivery van, etc). You should be able to find a different ratio for a GCWR downgrade.

A couple of vendors offer add-on overdrive/underdrive systems, and in some cases know what they are doing.

How many miles per year are you going to travel? The cost vs. paypack on gas mileage might not be worth the change. If you have replace the tranny then maybe it might pay back. I just had the tranny rebuilt on my SUV and it was $2700 and if I was going to do that on my MH it would take a long time to payback. I would leave as is JMHO.

Wit $$$$ You can put any rear end gear you want. Check with local rear end shops.