1) they'd have to COMPLETELY fix the disappearing vehicles in order to implement it
2) it'd have to be unacceptable as a horde base (slower than zombie running speed or something)
3) it would probably have to have a set amount of storage that couldn't be added to...like 2-4 boxes in preset spots and you couldn't actually add to the interior of the RV
4) would you make it able to be picked up and carried? That seems kinda silly for a mobile base. On the other hand, if you didn't, it's WAY too easy in this game for your vehicle to get hung up on terrain...possibly permanently making your base useless (it bounces onto a boulder in a trader-zone).
Honestly, I'd LOVE a mobile storage and crafting area (or them to fix vehicles enough that I wasn't rolling the dice on disappearing vehicles every time I left one outside a POI) so that I didn't have to make so many treks back to my stationary base and could more easily roam further.
Or they could just increase backpack storage, which would solve the problem completely for me.