First question: Does your MCI have the aux tank in the front baggage bay, thus giving you 179 gallons of fuel?
If so, seriously consider just running your genset off the dual OEM tanks and install accurate fuel gauges.
If not, second question: Are you keeping the OEM HVAC operational?
If so, ignore this suggestion.
If not, then here's what a fellow busnut I know did:
He installed his genset tank (about 40 - 50 gallons or so) in the old HVAC compartment, behind the main fuel tank. What was ingenious, tho, was the way he plumbed the darned thing. And with no filler neck to the outside anywhere!
Standard pickup and return out of the genset tank for the diesel genset, simple.
As you know, you've got a return line from the engine dumping excess fuel back into the main tank. What he did was to change that to a "T" fitting, so when the main engine was running, half the return fuel went to the main tank, half went to the genset tank. At about the 90% full point on the genset tank, he installed an overflow line back into the main coach fuel tank. (The genset tank was also vented over into the main tank.) So while running down the road, the genset tank was automatically being refilled, and once full, excess fuel would simply flow back into the main coach fuel tank via the overflow line.
But here's the slickest part of this setup: Many busnuts will install a small electric fuel pump in parallel with the main fuel line to the engine compartment to help reprime should they be fuelish enough to run out. This fellow went a couple steps further. He plumbed his extra fuel pump into the genset tank
with several 3-way valves, thus giving him the following options that I can remember:
1. Reprime the main coach engine.
2. Reprime the genset engine.
3. Transfer the genset's fuel to the main tank for emergency use.
4. Transfer fuel from the main coach tank to the genset tank for emergency use. This pickup was installed in such a way that it would always leave about 25 - 30 gallons of fuel in the main tank, enough to get to most fuel stations.
I've often thought this was one of the most interesting genset fuel supply solutions I've ever seen: simple, clever, and effective.
Don't know if this would work for you, but it's worth pondering!
FWIW & HTH. . .