Gear vendors expensive???
Well lets see - rebuilt 700R 1000 bucks, Torque converter 300 bucks, Crossmember mod 300 bucks, Linkage mod 100 bucks. If you have the parking brake on the trans big bucks!! Wiring mod for lockup converter 100 bucks. Drive shaft mod 250. And when you are all done you only have 4 speeds and they recommend you don't tow in overdrive whereas with a Gear vendors you get 6 forward speeds plus you can split gears going up hill. It's cost is around 3200 bucks.
I also build Hot Rods so here is my input. Just rebuild the 454, Beef up the T400 and add a gear vendors OD. Put headers on it. Depending on where you live there would be some concern as to smog requirements.
I think that when the coach has the switch installed from the factory you are set up for a 30 amp service. The OP said he only has a 6.5 K gen so to me that indicates a 30 amp service. Our 94 Pace Arrow has no switch and a 7 K gen set but is a 50 amp service. I think in order to rewire for a 50 amp service would be quite costly. IMHO
Take the radiator out of the system. Use a large aux cooler with a fan only!!!. My 94 Pace Arrow with a F53 chassis the tranny cooler just barely gets warm when the fan is on.
Also I have a Gear Vendors OD and do not use the factory OD.
Our Pace Arrow has a Solar Charger. It will keep the house batteries up to snuff all day long. That being said My coach batteries would go down in 4 or 5 days so I took some #10 wire put some clips on the ends and now I tie the coach and chassis batteries together when ever I boondook or am parked in a campground for more than a day. This system has worked well for 2 years.
It is amazing to me why this question gets posed. If you can afford a motorhome why would you worry about mileage?? Its going to weigh around 17K to 22K for a gasser and 30 to 40 for a DP. Any thing over 8 for a gasser is great and anything in the 10s for DP is good. Just sit back and enjoy it.