An UPDATE for anyone who may be considering a similar solution for creating occasional extra sleeping space ...
We decided to go with a small 2-person backpacking style of tent as this will be used for sleeping only and tent height or a spacious interior aren't a concern. I really liked the better quality tents by Big Agens, MSR, etc, but considering the limited use this tent will see couldn't justify the cost so I instead went with a moderately priced $140 Yanes Sentinel tent with all screening on the two long sides with fabric end walls, an entry/exit "door" on each sidewall, and a full coverage fly. It took perhaps 15 minutes to initially put it up on the deck yesterday but now that we know how it goes together I'm sure it can be done in about 5 minutes as there's really not much to it - tent, fly, two long poles, and one short pole for the fly. Anytime we choose to park it somewhere on our campsite I have a perfectly sized heavy duty 6' x 8' tarp on which it can sit, if we park it under the trailer awning it'll just sit on the under awning ground mat. It's 50" wide so it would if necessary accommodate two 25" self inflating mattress pads which pretty well occupy the entire tent. Thinking I maybe should have gone with the larger 3-person version I asked my 19 yr old son who will be using it if he thought this would be too tight for two people ... his answer was "Not for me and a girl!". Geesh, I walked right into that one! :S
Since we're not using this for backpacking size and weight aren't an issue so we chose a larger self inflating mattress pad measuring 76" x 25" x 3" - one for now but we can always add a second later if needed. The tent, mattress pad, and tarp roll up into quite small packages which as it turns out store easily in an ottoman we have sitting at the foot end of the bed in the trailer - out of sight, out of mind, but instantly available anytime we may want to create extra sleeping space without cluttering up the interior of our rather compact couple's trailer. Our next trip out will be the acid test but I expect this solution will work exactly as planned. :B
Thanks to all who contributed to this discussion! :)