A Lance 845 built after 07 should fit the bill. I say after 07 because they went to a metal frame and lost some weight over previous models. I have an 06 and it works great on my short bed 2500HD. I coldest I camped in in it was about 10 degrees and it did fine. I have two batteries,group 31's, mattress warmers and a portable generator if I need to charge the battery's.
If you want to pull a boat you should upgrade the hitch because the OEM hitches are not the best.
You should upgrade the suspension with shocks, Super Springs or something else you might come across. You need to upgrade to "E" rated tires.
For heat in cold weather I have a Wave 3 heater that is direct connected to my LP system. Some of the older Lances came with window inserts for winter camping. If you find one without, you can improvise . With Reflectix or something else. You will have to watch you tong weight for sure.
As was stated your brother will have to build a frame to keep the TC from sliding forward.
There are other short bed rigs out there you just have to look on RV Trader.
