And the coach I am investigating has Ring exhaust manifolds, as a non cracking replacement for the stock manifolds.

Are the Ring manifolds less restrictive than the stock 454 manifolds, or should I yank them and replace with Thorley's?

Early 454s had a reputation for cracked manifolds, especially in RV applications. The Ring manifold was an aftermarket design, ostensibly cast from materials less subject to cracking. I have no personal experience, but I suspect they were nearly straight knock-offs of the factory design.

Here's what erstwhile RV performance guru for Trailer Life magazine, John Geraghty, had to say some years ago about 454s. "Headers do not noticeably improve power in the RPM ranges usual with RVs, and they will usually cut mileage a bit. However, dual exhausts are needed and pipe size should be 2-inch minimum, 2.25 inch maximum."

He thought the Edelbrock Performer was the only worthwhile manifold change, but didn't think it was cost-effective.

He counseled against changing the stock cam.

I have found his advice worthwhile on several occasions. The usual tuner goes for performance car specifications which ramp up the torque and horsepower curves to levels inappropriate for RV applications, which usually demand stump-pulling torque at low RPMs. That's one reason why diesels are so desirable.

Geraghty's tuning advice was always directed toward improving low-end power.

John has done some dyno work for me, and was always good. His book is a treasure.

However, I disagree with him on headers on P30s. I have used them on previous motor homes, and have Thorleys on my pickup. I do agree with him on 2 1/4 exhaust pipes, but Thorleys have 2 1/2 collectors, and mine is a 502, so maybe 2 1/2 is optimal. So I went with the flow, so to speak. I also agree with him on the Performer intake, and will do it fairly soon. It is possible that, when he wrote that, he was speaking with experience with the standard hot rod headers, not the long tube or tri-wye headers like Bank$, Thorley or Gibson.

Here is a post from Larry in Colorado detailing the improvements on his P30: (I have done about the same thing, but he measured and I didn't. Plus, I live in the city and can't do easy hill testing)
.............................................

GM P30 Performance Improvement from Larry Reed

Note: Modifications performed in stages.

Vehicle: 1988 32' Bounder on GM P30 Chassis

Test Conditions: Hill climb, paved road, 7000' Elevation, 6% grade, .5 miles

Test Technique: Start hill @ 50 MPH, full throttle and manually shift for best speed

Metric: Speed at top of hill

Stage 1: All stock, new plugs, wires, carb adjustment etc.

Stage 1 Result: 43 MPH 2nd gear, speed still falling


Stage2: Added self fabricated air scoop*, 2.5" dual exhaust w/HP mufflers, cross-over-pipe $350

Stage 2 Result: 48 MPH in 3rd gear, speed still falling


Stage3: Added Doug Thorley Headers $600
Stage3 Result: 53 MPH in 3rd gear, speed holding steady
Note: Stage3 allowed acceleration up the hill


Stage4: Added Gear Vendors Gear Splitter $2700

Stage4 Result: 58 MPH in 2nd and Over, Speed holding steady
Note: Gear Splitters effectively double the number of forward
Gears. The benefits are obvious in many ways but are not fully
appreciated until you have tried it.

*I made the air scoop from a 2.5"x14" furnace floor register and 4" round flexible furnace ducting. I removed the silencer/flash suppressor from my air cleaner and connected the 4" duct to the air cleaner snorkel. The scoop hides easily behind the motorhome grill out of sight but in unrestricted air flow.