Hi Phil,
I think you could special order the largest furnace available. But oversize may lead to rapid cycling. The duct work might need a (difficult) upgrade for the larger size to cycle normally.
The only time having a bigger furnace would help is when warming up from stone bone cold. That is an unusual occurrence.
There is a hotel next door to my apartment condo, so I may ask if I can park and plug in, for a fee, in the future.
In the last year I've filled the propane tank twice. Mostly used for running the fridge. The tank is fairly small.