Hi, the gear selector shaft coupling may have come loose underneath, it's about level with the battery, midway down the underside length of the van. It looks just like a battery lead clamp on a hollow shaft that couples with the extension rod for the gear change using a serated shaft & the clamp.
I had it come off on mine going through Doncaster a few month back & I couldn't get it into any gear at all & had to do an emergency 1st repair to a Westy I'd only had a few weeks. If it's just come loose it may have rotated on it's shaft which makes it difficult if not impossible to get into certain gears(namely 1st & 2nd). I quickly diagnosed the problem when it happened to me & hurriedly reconnected it. It got me home but I had to reposition the next day so I could get 1st & 2nd gear easier. I guessed it's position using 3rd gear slection on reconnection & it's been ok ever since.
Have someone in the cab, split the coupling & keep hold of both ends. Get whoever's helping to select 3rd gear whilse your still holding both shafts, then carefully line up the main extension shaft & slide it into the hollow shaft as far in as you can. Then tighten the clamp & give it a run out to check.
1st is now a very deliberate select on mine, but all the other gears are fine to select. I may have the positioning slightly out & if anyone has a proper method, please don't be shy about it
Hope this helps.