You don't say what chassis you have, Ford ? I did a lot of measuring before I cut the hole in the floor. The fuel pump on ours turned out to be about a foot outside the bed, the only problem I encountered was the heating duct runs down the middle of the coach and I needed to cut through it also.
After the "access door" was cut it took about twenty minutes to actually change out the pump. Being as our HR is on a '91 F53 chassis I also had to upgrade the wire shunt that plugs into the new pump. If I remember right the pump was about $350 and the wire shunt was $25 from RockAuto.com.
92' Holiday Rambler 1000
The wife, me and two furry kids
If you have it, a truck brought it.