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I need a 50 amp RV service but my 200 amp Siemens panel is full. Can I install a Siemens 100 amp sub panel in the unfinished stud bay next to the main panel using tandem breakers to make room for the feeder breaker? If possible what feeder breaker and wire size? Would the wire need to be run in conduit? I was thinking that adding the sub panel could supply the RV and possibly some lighting circuits in the future?

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    You should probably move a whole bunch of circuits so that the original panel has some breathing room - shouldn't need any tandems, and by using the same brand/type of panel you get to move breakers/circuits between the panels as needed. But a proper answer will depend on pictures of your existing panel.
    – manassehkatz-Moving 2 Codidact
    Feb 13 at 5:25
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    Can you post photos of the existing panel please? We might be able to find ways to find space in it, or expand it for that matter
    – ThreePhaseEel
    Feb 13 at 5:25
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    Do you actually need 50A RV service? Or do you want a 50A RV receptacle to use for EV charging? Because (a) most people don't need that much power for EV charging and (b) hardwired is often a much better option.
    – manassehkatz-Moving 2 Codidact
    Feb 13 at 5:42

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Since you're at this point (full panel), get a BIG subpanel (many spaces) and if that means the sub-panel is also 200A rated or has a 200A main breaker installed in it - no big deal - the 100A breaker feeding it will rule - if your service can support the loads you are adding per the load calculation you need to do, without a service upgrade.

Then, indeed, move breakers and their neutral wires to the new panel, wholesale, rather than wasting money on Tandems to replace full-sized breakers. A minimum of one double-pole or two single-pole breakers moved will get you space to feed the sub-panel. If you (now or later) need to upgrade to a 400A service, a pair of 200A main-breaker panels would be just right. But if your load calculation permits, a sub-panel can be vastly over-rated, because the load on it is controlled by the breaker that feeds it from the main.

Provide several large diameter, short, (preferably) metallic conduits between the new and old panels to make moving circuits between them easier.

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