Plan your stops in advance. About 100-miles/2-hours apart for 10-minutes. Fuel & food at 4-hours to be out of the vehicle one hour. It isn't just best safe practice (truck industry), it also is the preservation of fine motor skill. Keeping the vehicle lane-centered is nearly as important as chosen travel speed, in a manner of speaking.
By knowing in advance the stops (and where, specifically one is headed at low speed as in across a parking lot) one loses only a few tenths from the overall mpg. I try to keep my choices to,  the direction of travel,  past any intersection that would be between me and the IH entrance ramp,  that affords ease of entry and exit. Etc.
I also prefer stops at the periphery of metro areas (unless lunch, let's say, iis planned at a highly-regarded local eatery), both before and after. An after-meal restroom stop should be included inside the first
hour after departure.
A trip broken out into legs of time/distance with known stops is one where it is much easier to keep a higher level of attention en-route. Avoid random stops altogether.
And, traveling at 50-mph would be rather stupid, IMO. Too much energy watching the mirrors for overtaking traffic. Try 57-58 and keep the center of your chosen lane. (If you move to the right of the lane, the passing traffic will nearly cross over to your lane . . keep 'em bumped over to theirs, if need be, by judicious use of the left of your lane. Most all drivers are unconcious of how they make choices. Include yourself in this: always have 4-seconds separation between you and others, no exceptions?
. . The goal is to keep the energy expenditure low as is reasonable, both driver and vehicle).
Good luck, and have an enjoyable trip