Already asked--30 to 50 amp

You'll be fine with the adapter. I think the manual is doing a cover our rear end type of disclosure. Reasons why is below but probably won't make much sense.

I'll probably get beat up for this because I'm not sure I can adequately express what I'm trying to say. But In simple terms the only difference between a 30 amp hookup and a 50 amp hookup is the shape and number of slots on the plug. The plug by itself does not limit the amount of electricity that can flow through it. The circuit breaker that feeds the plug is what limits it to 30 or 50 amps.

Let me try this a different way as it suddenly came to me that this might make more sense. This is hopefully a very simplistic easy to understand analogy.

Picture your RV as a big empty cardboard box with an air conditioner sitting on top of it. Now you want to run that air conditioner so you go to Home Depot and you buy an extension cord and plug it in. This in essence is your 30 amp hookup.

Now you're still hot so you buy a second air conditioner and put it on top of the box. You go to Home Depot and buy a second extension cord and plug it in. Now you have a 50 amp hookup.

DISCLAIMER HERE: No you cannot really just plug two Home Depot extension cords in and you have a 50 amp hookup, I'm just trying to convey a mental picture. :)

Now instead of two cords going to the RV, picture that they both go to a 50 amp plug in the power pedestal for people to plug into.

If you notice on your adapter there is an extra blade on the 50 amp side of the plug. Think of this extra blade as the second extension cord that you used for your second air conditioner. All your adapter does is lets you plug into the 50 amp outlet, but it never connects the second extension cord anywhere. So essentially you have one extension cord powering your RV and the adapter has disconnected or unplugged the second cord at the wall.

Don't sweat it.
Starcraft probably has it written, "Connect shore power cord only to 30amp 120V receptacle". That is a two-fold admonishment: only 120V (not 240V) and for full functionality of the trailer 30 amp service is required (not your garage 15 A circuit).

Here's what happens when you use your 30 A adaptor on a 50 A pedestal: There are four wires to the 50A pedestal receptacle. Two of these wires are 120V powered, one neutral and one ground. Your adaptor connects to one leg of the power, one neutral, and one ground.
Your RV's 30 amp load center will limit the draw of any electrical devices to 30 amps (not really that precise as the main breaker has it's own specification but accurate enough for this discussion).

I would suggest you buy two things, a handheld multimeter and a 120V circuit tester that has the small lights and will alert you to potential power source problems. Both of these can be had for less than $10.

If your starcraft has a 30amp main breaker on your panel, then you are fine. Anything over 30 amps will trip that breaker. If, for some strange reason, you don't have a 30 amp main, then don't use it. My friend's coleman popup has a 30 amp plug that has no main breaker. Just a 20 amp for his ac and 15 amp for his outlets/converter. In reality, he could draw 35 amps on his 30 amp cord if he is connected to a 50 amp outlet.

You should not have a problem using a 50 to 30 amp adapter.