SUCCESS!!!
I managed to get my loose right wheel back on the axel snug and tight without the wheel falling off!! Both back wheels on my Tumbler are now secure on the axel. The way I did it was to pop off the center cap on the loose right wheel. Here's a picture of the right hub cap and the metal axel nut that secures the wheel to the axel.
Photo of the left wheel with exposed axel nut.
Here's the way I got the center hub cap off the loose wheel and secured the wheel back on the axel. First lay the loose wheel on a flat surface and place the tip of a flat head screw driver (or any other hard object that will fit inside the hole on the back of the wheel) directly on the back of the nut inside the wheel. Then take a rubber mallet or hammer and give a couple of forceful taps against the screw driver handle top. Doing so, the force of the hammer against the screw driver will cause the nut to press against the backside of the hub cap which in turn the hub cap will pop off. Also, the nut will fall out of the wheel. From what I could tell, there doesn't seem to be any glue holding the hub caps in place. If there is, it's sparse and not visible. Once I got the hub cap off, I had to pull the exposed axel toward me a little so it could be more flush against the inside of the right wheel to accept the tip of the nut that fits snug inside the hole of the axel. I held a flat tip screw driver against the nut on the opposite left wheel (hub cap came off in the box when delivered) while I pressed and turned the nut into the axel on the right wheel with another flat head screw driver. Now both wheels are snug and tight on the axel. No more loose wheels!
Putting the hub caps back on is not that hard. I just took a screw driver, which I covered the end of the screw driver handle with tape to prevent it from messing up the paint on the hub cap and tapped once and/or twice lightly with a rubber mallet to secure both hub caps back on the wheels.
Thankfully I don't have to worry about a response from HT CS, which I haven't received yet or contacting CSC for a refund or exchange. Now to find some time to put the wings/flaps on my Tumbler.
I hope this procedure helps others with the loose wheel issue.