1986 Allegro 32 ft class A 454 GM engine T400 trans 4:56 rear. It has good guts on the hills for a non-diesel.
We get around 7MPG @ 60 MPH. Keep it to 55 without the trailer behind it and you can eek near 10 out of it on the highway.
As it gets closer to 75MPH, the mileage can go down to *cough cough* 3.5MPG.
We got ours 5 years ago with 11,000 original miles for $4,000.
We spent more than $3500 going through tires/hoses/belts/brakes/synthetic fluids in everything bumper to bumper...etc. 19 inch tires were unGODLY expensive.
It now has just south of 30,000 miles on it and burns zero oil, runs perfectly and has only needed a fuel filter, ex manifold gasket...and a cracked spring presently needs to be replaced in the rear.
Fuel injection is nice and has advantages.
An old carb can be fixed by any mechanic.
The whole HEI ignition system for mine has a spare in a box that I can change, from inside the rig, on the side of the road...in less than an hour myself.
After very careful pondering, to us, keeping it simple was paramount for the long haul.
It is not shiny. The paint is faded and the interior fabric, through in great shape...is very dated.
No overdrive either. She "sings" on the highway @60. Above 65, the singing turns into "screaming."
That said, we tow a heavy car trailer with lots of tools and a station wagon on its back. That 4:56 rear is needed in our scenario. Tradeoffs are inevitable.
The 460 Ford engine does well in the pickups.
Not so much in RV's. Not sure why really, but the numbers are the numbers. The fleet operations, as a rule, stopped using them in the step van chassis for this reason.
Also, check out how the 460's in those "van nose RV's" tended to burn them to the ground. A good drive train to avoid in the RV. I would stick to the 454.
(The same does NOT go for the lighter duty units with the "302/351" engines, which have great records of service by comparison.)
You get most of the same issues with a 12 year old unit as with a 30 year old unit. (Drive-train wise.) Most of the same stuff dries out and needs attention.
The 30 year old GM unit however, can be made to run flawlessly (as long as the engine is compression tested/oil sampled and found to be healthy) with a $150 dollar HEI unit and a $300 carburetor.
That is the entire system to make it run. Simple is good. Simple is cost effective. Any mechanic can fix it...and no computer needed.
As for "units that sit." They all sit. Mine sat, un-started for more than 18 years. If it starts up and runs well after 500 miles or so, it will likely continue to do so throughout its normal life cycle of mileage.
People warn of seals drying up. This is true, but it does not always happen. When it does, it means you will have an oil seepage. Depending on where that is, (front crank seal/rear crank seal or valve covers) dictates the $$$$ involved to remedy it. A rear main seal requires the transmission to be removed in order to fix it.
That said, we are talking about (90% of the time) a few drips here and there. It is not like you would HAVE to fix it...nor does it leave you on the side of the road. (The same cannot be said for the multitude of sensors and aged electronic connections...in-tank fuel pumps, etc etc that the 90's and newer units depend on to run. Any one of those fail and you stop. No passing go, no collecting your 200 dollars worth of RV fun.)
We have two policies for insurance.
One for the inside (agreed value) and one for liability. The liability policy is 320 per year and it gives us 500K of coverage.
(I think it is 500K if we hurt someone and 150K property damage.)