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The longest palindrome I’ve ever seen was “A man, a plan, a canal; Panama”.
Yeah, but like it says in that article, the palindrome in question is nothing more than a series of words strung together - they make no sense as a unit.
“A man, a plan, a canal: Panama” makes actual sense.
Ah well, as Napoleon said, “Able was I, ere I saw Elba”.
As much sense as a verbless sentence can make. 
Wow.
Go hang a salami; I’m a lasagna hog!
Dad, step aside - Ed is a pet’s dad.
Have you ever heard of this nonsense word: drowesnesnonsihtfodraehreveuoyevah?
Oh, I can do better than that, RM!
“Have you ever heard of this nonsense word: kozargluflenbobvcaxacvbobnelfulgrazokdrowesnesnonsihtfodraehreveuoyevah?”
Try these:
Are we not drawn onwards, we Jews, drawn onward to new era?
Now ere we nine were held idle here, we nine were won.
Now saw ye no mosses or foam, or aroma of roses. So money was won.
And the magnificent:
Dennis, Nell, Edna, Leon, Nedra, Anita, Rolf, Nora, Alice, Carol, Leo, Jane, Reed, Dena, Dale, Basil, Rae, Penny, Lana, Dave, Denny, Lena, Ida, Bernadette, Ben, Ray, Lila, Nina, Jo, Ira, Mara, Sara, Mario, Jan, Ina, Lily, Arne, Bette, Dan, Reba, Diane, Lynn, Ed, Eva, Dana, Lynne, Pearl, Isabel, Ada, Ned, Dee, Rena, Joel, Lora, Cecil, Aaron, Flora, Tina, Arden, Noel and Ellen sinned.
The best Ive ever seen (not the longest, I’ve seen longer but they didn’t make much sense) was:
Doc, note, I dissent. A fast never prevents a fatness. I diet on cod.
Great, isn’t it. Sounds like a perfectly natural saying, you could imagine someone actually saying that. With most long pallindromres the sentence is unnatural.
The usual term is reversal. However, a number of people (the first, I believe, being Lewis Carroll) have come up with the clever name semordnilap for them.
Re: Napoleon’s words. Later on he was heard to say “An elehts was I ere I saw St. Helena.”
And why doesn’t anyone remember the earlier epigram: “A man, a plan, a canal – Suez”?