Any main breaker panel will suffice
All you need here is an outdoor (NEMA 3R rated) panel with a main breaker and a few breaker spaces. Your power comes into the main breaker, then goes out to the various buildings you want to power.
Normally we recommend a "Ranch Panel" which has a 200A main breaker, 8 breaker spaces for 4 feeders to outbuildings, then "thru lugs" to carry full 200A to the main building. However I have an intuition that you are cost-conscious, and the existing wire to the abandoned trailer house will never carry 200A anyway. Therefore a regular old panel will suffice.
Harper's suggestion: phase 1
I would do this: I would choose a 200A main-breaker panel. Since your electric service right now is 100A, I would install a 100A breaker* in a normal breaker position, with a tie-down kit. I would wire the utility power into that 100A breaker and just completely ignore the 200A breaker. Take care to label the 100A breaker "MAIN BREAKER" since that will not be obvious.
Then, a 50A breaker goes off to the camper stand, straightforward stuff.
Now, how do you power the shed? Since the wire to the shed is 100A rated, all we really need to do is connect to the breaker bus. How do do that for free? Another 100A breaker costs money. So do subfeed lug kits. But look! That unused 200A breaker has two lugs on it, and they directly connect to the bus! (via a 200A breaker that will never trip). So there you go - connect the shed to the 200A breaker. "That was easy"
Harper's suggestion: Phase 2
When you get to the happy? miserable? day that 200A service is actually required for your needs... What now? Costly new equipment? NOPE.
Remember when we previously flipped the direction of the 100A and 200A breakers? We flip them back to normal. Now, you attach your 200A (4/0 cable) service entrance wiring to the 200A main breaker. The shed wiring then moves to the 100A breaker. The camper wiring remains on the 50A breaker. Zero cost except for the new 4/0 wire to the weatherhead.
* Normally I wouldn't recommend Square D QO as it is the most costly panel on the market. However, in your case, your picture appears to be showing us a totally free Square D "QO" 100A breaker you can use. QO is very good stuff, and it's probably fine.