That would be really tough to retrofit in. You would have to rip out the drivers area walls to build the supporting structure, if yours even is capable of that. You also would have to assure that your modifications did not weaken the steel cage used for rollover protection. I doubt any vendor of such a bed would sell it to an end user due to the liability if it was improperly installed and it fell down on the driver while moving.
This is the electric one we had factory-installed:
https://www.lci1.com/euro-loft-bed-lift-by-project-2000
Time to get her to approve buying a new motorhome, I guess.
Good luck,
Ray