Humans have been producing social meaning from language nearly as long as we've been producing excremental material from nutrition.
We might not need many mental steps to move from buttload to shitload and, from that juncture, produce a picture, feeling or perception that some would find vulgar, inappropriate for some situations, or with which we might otherwise sit unpleasantly.
Butt, in and of itself, for example in (someone/something is) a pain in the butt or give (someone) a kick in the butt seems relatively less likely to offend on the sensabilities of most native English speakers than might a buttload, probably because reactive to butt, we'd expect even the more imaginative among us to conjure up an image of the prototypical fleshy globes so visually similar to peaches (a perfectly wholesome fruit), and often associated with expression of cuteness, humor, and a supple vitality. If any other sense would be generally activated consequent to such a crack, anecdotal evidence (introspective case-study method) suggests it is likely to be through the aural perceptive modality, such as the good-natured thwack emanating from a locker room in which happy and well-paid athletes demonstrate charming and harmless fraternal bonding via various playful and congratulatory smacking and patting maneuvers, or the popular songs of Jennifer Lopez (a 1997 Golden Globe nominee).
With buttload, on the other hand, fundamental psycholinguistic principles plainly suggest cognitive focus would often receive a dump of emotionally charged information squatly bound to the load component of the term, suggesting--even forcing--reactivity to foulness or an intolerable reek, funk, or dirtiness that may be more likely to cause a negative reaction in some than the use of the term butt in itself.
The term load, as betrayed by its common transition from noun to verb, represents potential action, a promise of release which most may yearn for, at least symbolically as a projected residue of past experiences with struggles against letting go, but which few truly wish to experience intimately as a product of a constructed other.
There are a shitload of terms and phrases to convey the idea of a large amount. Many people find vulgar terms, profanity, sex-related terms, or potty talk useful because they often carry a striking intensity, or deliver a powerful emphasizing effect, which may be why you chose shitload as a starting point for your question rather than merely asking for strong or emphatic terms that mean a large quantity.
You might use mountains of (substance), or millions of (objects) effectively, depending on tone of voice, other contextual factors, and exactly what you might be referring to.
If you supplied a particular example, or specified the question further, we may be better able to supply what you're looking for.
Meanwhile, searches in dictionaries and thesauruses for synonyms for simple terms like many, myriad, large, massive, huge, ton, etc., should provide a range of options. If none seem to do the trick, you might try editing your question and trying to explain what you want to express that such alternatives don't seem to accomplish.