I'm going to try and ignore the fact that he's a recovering alcoholic for a moment...as that puts a slightly different perspective on things.
Firstly, you can buy them in packs of two. What was the cost of the Viagra on the receipt? this would indicate how many he bought initially and whether any were used. i used to get packs of 8 for £28 online, so roughly £3.50 per tab.
Secondly, for all those saying no man would keep them in his wallet - Don't forget PDE5's take 30-60 minutes to kick in. Therefore, having them in his bedside table is not all that practable as by the time he takes one, upstairs, already half in bed, he won't have the time for them to do their magic. A wallet is usually in a mans pocket, or close by so they are therefore easier accessible.
Men get erection issues at some point in their life, and most men don't talk about them. Keeping it a secret from you is not a sign of infidelity IMO. Has he had issues in the past?
If you have been relatively sexually inactive for some time, it is likely he is still masturbating, and he has possibly had issues getting fully hard. They may or may not be for his own use during his 'personal time' or, may be to give him an extra boost either with or without you.
Unfortunately your trust has previously been broken with his alcoholism and lies, most with an addiction become compulsive liars and learn to cover their tracks. Now, being a RA does not mean he will cheat on you, they are and should not be linked in any way, but, you will likely find it difficult to tell when he's lying and telling the truth.
You need an open and frank conversation with him, don't jump to conclusions but let him know you want the truth and nothing but the truth.