Depends on what MCBs you are having. A 16amp would be fine if you are having a 6 and 10 amp MCB as is standard. But a 25amp is probably the norm TBH. 10 amp RCDs are quite unusual and most wholesales I use wouldn't stock them.
RCDs work differently to MCBs. RCDs work by detecting an imbalance in the live and neutral conductors, such as can happen when there's leakage to earth. An MCB detects short circuit or overload. So the first line of defence for Overload protection is the MCB. The amperage of the RCD should be slightly greater than the overall load for the system, as the RCD also has overload protection.
Actually buying a 16A RCD is expensive so go for a 25A..
The Sargent EC155 unit in VB uses a 40A RCD but the current is fairly irrelevant, so long as it is sufficient, as the wiring and equipment are protected by the MCBs. As noah says the RCD also has overload protection so slightly above your full load is good.
I am not sure that you need a separate main isolator as this is an extension of the fixed installation that already has one. Otherwise as it is local you should be able to rely on the RCD for isolation should you require it. Anyhow I am sure for most of us the isolation is a big blue plug.
The hook up upstream supply will determine / protect what is available so it will be fused at 16A or less. Ultimately this is the overload protection for your hook up cable and so your installation and equipment. I believe the bollards at CCC sites have both MCB and RCD or possibly RCBOs.
Just go for RCBO's they are more expensive but less annoying if anything trips as it only takes out the circuit with the fault on it. They are starting to make RCBO's the same size as mcb's to make it easier for terminating in consumer units. Any time I need to provide rcd protection on circuits (domestic dwellings) I always advice on RCBO's.
You could use RCBO's no problem, but you still need to provide a main switch which disconnects all live conductors. If you only have one circuit then the RCBO is fine but with two you will still need a main switch.
It's interesting reading the regs in full. I doubt many installations, even brand new coach builts comply fully.
And as for hidden hook up points under vans and under bonnets etc, well that's a huge can of worms.
Yes a double pole main switch will be needed and rating above the maximum load on the board. 100amps are the most common. They don't act as an mcb, they are strictly for isolating every circuit in the fuse board