Cheating in Ultima V

Ultima V has fewer cheats than the later Ultimas.

Maxing Stats
When you die, you lose 50 exp.

So get your characters to around 220 experience, camp until Lord British appears to set everyone's level to 3 and then die. You will go back to level 2 but you will NOT lose the stats (INT/STR/DEX). Repeat the process until you reach 30-30-30.

The results are amazing: your party will be overwhelming. I think DEX is the most important attribute, as you will get to take several turns before your opponents move and also you will never miss an attack.

More Reagents
In the C64 version, if you have between 11 and 15 of some reagents, then mix a spell using only those reagents, and mix 16 of the spell. You should get 16 of the spell you mixed and 90+ of the reagents.

More Food
Clever Avatars can use a bug in the C64 port.

In the pub of Empath Abbey are provisions where the programmers have forgotten to apply a "other people's stuff"-tag. Meaning: you can take them and it's not stealing. Climbing then up and and down the ladder exploits another bug, as the food will re-appear. This means without paying or karma-loss you can get lots of food.

Castle Britannia's Bottomless Vault
At the bottom of Lord British's Castle is a magically locked door, beyond which can be found three chests that are always filled with goodies. You can find basically anything in these chests, including the valuable Magic Throwing Axe. These chests respawn if you go up-and-down stairs, but that also relocks the magical door. The best way to revisit the chests without having to waste a Skull Key every time is to sleep for 1 hour. There are 2 beds on this level of the Castle, so make use of them when they're not occupied!

Infinite Skull Keys
Kristi in Serpent's Hold will sell you 5 Skull Keys for 100 gold. So long as she never asks you for your name (prompted by saying 'visitors' to her), you can keep asking her to sell you more keys.