I think
Gumby might have a difficult time getting into the driving seat due to his unusually large feet.
Imagine Gumby trying to shove his big green feet between the
firewall, the dashboard, the seat... the motor would be idling and Gumby's big green feet would be pushing switches and buttons and things...and Pokey would be telling him to be careful...
If Pokey were to drive, his legs would slide in easier due to their smaller cylinder shape. Pokey could drive and Gumby could ride in the cabin area seated at the couch, diner or maybe in the overhead bunk watching TV???
They would be eating and driving and talking about where there were going to go and what they were going to do...
Gumby would be wanting to take risks and chances and Pokey would be very conservative and cautious...
The best floor plan would be for them would be to halve the front half with the seating, diner, kitchen, shower and then the back 1/3 could be for Pokeys stable area, where Pokey would go and act like a horse at bedtime.
Sounds like a nice time. Traveling with two cartoon characters.